How accurate is your Mode C?

[Deeday]

I've received some conflicting advice from different instructors, with regard to leaving the transponder on ON, as opposed to switching it to ALT (flying OCAS on a light aircraft).
The argument against Mode C is that, with an accuracy of +/- 200 ft, we could happily be flying well beneath the Gatwick CTA and still be called up by Farnborough, who would see us as about to bust CAS.
Is it that the transponder on the aircraft I'm flying is particularly crappy, or is such level of accuracy what you can normally expect from a Mode C? if anyone knows. Thanks.

[LH2]

I've received some conflicting advice from different instructors, with regard to leaving the transponder on ON, as opposed to switching it to ALT

First stop would be the UK AIP. I don't know what if anything it has to say about this, but in other places, such as France, there is generally a requirement for squawking mode C if so equipped.

There was also that midair in LAX about 20 years ago, which should be as good an argument as any as to why not using a properly functioning mode C is a very daft idea.

The only time when it needs to go to ON is where the AIP tells you to, or when ATC requests you to do so (to avoid spurious TCAS RAs).

with an accuracy of +/- 200 ft

Where does that figure come from? As I recall it (I cannot get to my books right now) Mode C encoding resolution is 100ft (i.e., max encoding error +/-50ft), and altimeter tolerance according to CS-25 (again IIRC) is +/-30ft per 100kt CAS at MSL. By applying the relevant error propagation formula that results in a total error of +/-60ft. Granted that this is oversimplistic (e.g., not taking into account scale, position, mechanical or hysteresis errors) but for light GA purposes, a +/-200ft error such as you mention sounds a bit excessive, assuming your pressure settings are correct. In any event, this is still less than half the separation between standard levels.

we could happily be flying well beneath the Gatwick CTA and still be called up by Farnborough, who would see us as about to bust CAS

Shouldn't happen unless you're flying a non-standard level (which in VFR you legally could, but...) In any event, I see it as a bit of a non-issue--if you're going to be flying that close to airspace you're not welcome in, then at least speak to them and state where you think you are (vertically as well as horizontally) and what your intentions are, for everyone's peace of mind.

But to answer your actual question, mode C resolution is 100ft pressure altitude, QNE.

And in any event, I predict Bookworm will be here in no time to provide a reliable answer :}

[Johnm]

The tolerance is +/- 200ft in practice my Garmin is spot on with my altimeter within 50 ft.

ALWAYS display mode C if you have it

[Rod1]

You should keep it on ALT provided it is serviceable (accurate to +/- 200 ft) and you are not flying in close proximity to other aircraft, eg formation flying. Some older encoders do take some time to literally warm up, so if you have one you probably want to select ALT as you line up.

This has been done to death several times…:ugh:

Rod1

[DB6]

Verify it i.e. ask the ATC radar to tell you what FL you are showing on his screen. ATC do that anyway in order to reduce the vertical minima they have to apply between squawking traffic returns (less if verified).

[Fuji Abound]

This is one of those old chestnuts and reminds me when instructors use to tell "students" to switch off their transponders in the circuit or any where near a "big" airport.

From my point of view you would be absolutely nuts to do so. Some of us have TAS (TCAS) and love you to death for squawking mode C (or even A) in the first place.

It is quite possible with some transponders for the altitude encoder to be "out". I believe the rules have changed yet again on how often the encoder is checked and calibrated and are different between the G and N reg. regimes, however it takes all of 5 minutes to ask any unit to confirm your height. Simply say you want to to check your mode C and ask them to verify your altitude.

I dont see a great deal of point flying within 100 feet or at the same level as CAS, but if you do and your transponder was inaccurate, you have done nothing "wrong" in any event. If your "infringement" is by 100 feet AT will more than likely ask you to confirm your height or point out that you are "just" infringing. Clearly if you altimeter indicates otherwise and you have the correct pressure set you have not infringed and should only need to confirm your correct height and pressure setting. I have had a few periods with an "old" transponder that regularly "played up" and had a defective enconder. Although it appeared that I was at the "wrong" height it never caused a problem with AT and of course it was fixed although rightly they pointed out that the transponder was inaccurate and its use was certainly not to be encouraged within CAS were understandably it caused more than a little concern.

Take note that it is also quite possible that your altimeter may not be correct since it was last tested.

If you fly with TAS and aircraft are squawking mode C I can tell you the technology is first rate. Even with numerous aircraft in the circuit I can "see" every aircraft and it is a significant asset. Yesterday, just as an example, the circuit had just two in both squawking mode C and from 20 odd miles out when I first took an interest in the circuit traffic I could see exactly where they were and identify the aircraft asked by AT to orbit on final. Slotting myself into the pattern to ensure both aircraft had plently of room to complete their approach and the other to roll out was straight forward and I guess made life a little easier for everyone.

In short if you have mode C please do not turn it off in ANY circumstances.

[Rod1]

Fuji

“If your "infringement" is by 100 feet AT will more than likely ask you to confirm your height or point out that you are "just" infringing.”

With the new more “assertive” approach to infringements with the use of the automated system, are you saying it is set to ignore the first 200ft? If not then I was under the impression you got reported and that was that?

“In short if you have mode C please do not turn it off in ANY circumstances.”

Are you completely sure about that.;)

Rod1

[Warped Factor]

CAIT (the automated infringement alerting system in the London area) will highlight any unknown traffic with A/C that is flying at or above the base of CAS.

So if the base is 2,500ft, any unknown traffic with a Mode C readout of 2,500ft or above will be highlighted. Tracking along at the base level is very unlikely to result in you being imprisoned for infringing! Above the base level ATC may well have to start avoiding you as we might have traffic inside CAS at just 500ft above the base, but we're well aware that Mode C tolerance is +/-200ft. Better safe than sorry though.

See AIC 15/2007 Pink 112 (http://www.nats-uk.ead-it.com/aip/current/aic/pink/EG_Circ_2007_P_015_en.pdf) and AIP ENR 1.6.2 (http://www.nats-uk.ead-it.com/aip/current/enr/EG_ENR_1_6_en.pdf) where it is strongly recommended that in airspace where a transponder is not mandatory you should still select A and C.

[Fuji Abound]

With the new more “assertive” approach to infringements with the use of the automated system, are you saying it is set to ignore the first 200ft? If not then I was under the impression you got reported and that was that?


I think you are under the wrong impression then, but happy to be corrected.


Are you completely sure about that.


As some one else said there maybe some rare circumstances - flying in formation has been mentioned. Clearly if the transponder is transmitting duff information that is another good reason. However, again unless you know better, I can think of few good reasons for turning off the transponder.

[steveking]

Interesting stuff, I could almost think this thread was about me.

Last year I was coming back from Hull heading for Luton/ Stanstead gap. Whilst descending London info asked to immediatley contact Luton as I was about to infringe. I looked at my height, checked the QNH, looked at my GPS for the TMA lines all seemed fine, admitedly I was close but nertherless still underneath. Anyway by the time I left the gap the controller and I had worked out a 250ft error on my reporting height, this was down to a 60ft error on my altimeter and the rest on the encoder.

Encoder switch at 100ft intervals but may for example switch say at 2380ft indicating FL024 assuming 1013
to
2390ft indicating FL025 assuming 1013

Now if you add a 150-200ft error then you could be at 2390 but indicating FL 027.

In my case this plus my altimeter error didn't look good on the radar screens but although I did get a follow up call every one seemed happy with what had happened. Since then I have had a garmin 328 installed which very nicely shows the flight level that is being transmitted and my encoder has been adjusted and I am now within 150 ft transmitted height to actual height.

The controller also informed me that there software automatically adusts transmitted flight levels into QNH heights.

Anyway my little incident managed to make into the GASIL book which is why I think I got the follow up call so they had a few more details about the transponder error and for my part as others have said I try not to fly so close to CTA.

[Mark1234]

So, say for the sake of arguament, one is wallying along just under someone's controlled airspace, and elects drop mode C, and goes mode A only, what impact does that have on those controlling the airspace - do they just 'assume' you're outside vertically unless proven otherwise?

Personally I'm pretty much always mode A+C; we're only asked to have the transponder on SBY when doing circuits. No radar locally, but others nearby.

[mm_flynn]

So, say for the sake of arguament, one is wallying along just under someone's controlled airspace, and elects drop mode C, and goes mode A only, what impact does that have on those controlling the airspace - do they just 'assume' you're outside vertically unless proven otherwise?
Mark,

I see from your details you are based in Oz and my answer is related to UK and US.

First, you will frustrate the controller who will have yet another no altitude target (and maybe be illegal if in the US). If the controller notices you were Mode-C and now are Mode-A, I would guess he will then treat your target with care, assuming either you have a problem or you think you are lost/infringing and don't want to be caught.

You then become an irritation to everyone with a TCAS/TAS or similar system who still know of your existence but have no clue where you are vertically, finally you get a place in the Cussed Poor Airmanship list for being deliberately awkward.

[172driver]

Not sure if this is the case in the UK, but in some countries, e.g. Spain, Portugal, if flying within CAS, on initial contact the controller will include something like "XX-YYY radar contact, 5500 feet". A quick glance at your altimeter then shows if you see the same as the controller.

[PlasticPilot]

Mode C transponders have a 100 feet resolution, but if you have a Mode-S transponder, its resolution is 25 feet. Since introduction of Mode-S we often see airliners crossing a whole FIR at FL301 (30050 feet)...

Buzzing exactly at the base of any airspace is anyway not a good plan, at least warn them you're here.

[Fuji Abound]

I cant help feel so much of this is also down to common sense and good airmanship.

Working AT should not be a them an us issue. If you have good reason to duck under CAS by 100 feet, tell the controller what you are doing. You might want to transit Gatwick under CAS east of their zone. Sure you can do so at 2,400 but it will take you directly below their IAP. Put yourself in their shoes, as much as you are entitled to do so without talking to anyone, wouldnt you be cautious about a pilot doing exactly that? If on the other had you have reported your intentions the controller they are more likely to think this guy knows what they are doing and if their altitude appears to be 100 feet out more than likely it is down to the transponder, altimeter or his being a little complacent about keeping accurate station. (more likely than not the last :)) All that is necessary is a firm reminder that 2,400 means just that, or perhaps you might like to adjust to 2,300 and no one need get too upset.

Now we all know that school who turn off mode C near CAS - I will not be caught now if I infringe - so the argument goes. Well you will potentially cause chaos, probably be prosecuted in any event on the basis of the primary return and be a right pain to those of us with TAS. It is just not clever.

[steveking]

Since my little incident I have took more notice of altitude reporting errors in the gasil and whilst on frequency with LARS units. It's not as uncommon as you would think, I have heard several times in the last few months of aircraft with reporting errors of several hundred feet. In the gasil I noticed one with an error of 5000 feet. Something for those with TCAS to think about if flying in uncontrolled airspace the mode c returns may not have been verified with a radar unit.

[Fright Level]

I believe a Mode C/TCAS altitude error almost caused a mid air between BA and Korean 10 years ago ..

BBC News | Asia-Pacific | BA near-miss inquiry (http://news.bbc.co.uk/1/hi/world/asia-pacific/391213.stm)

Safety-Critical Mailing List Archive 1999: TCAS near-miss: "an error in just one wire making it 1-0-1" (http://www.cs.york.ac.uk/hise/safety-critical-archive/1999/0158.html)

[ShyTorque]

From another long time TCAS user who has to use Class G every time we fly, please always use Mode C unless you suspect it is in error or ATC have asked you to deselect it.

With the help of our TCAS we very often see other aircraft that obviously have not seen us at all (I say that because many don't obey the rules of the air, with regard to rights of way - so we have to do the avoiding for them).

In the obvious interests of flight safety, we don't mind doing the avoiding as long as you help us in every way to do the seeing. Mode C is far better than plain vanilla mode A alone and helps us keep visually scanning the rest of the sky as well as the sector you appear to be occupying at "some altitude or other".

Ta.

[englishal]

Mode C references flight level.

Last time I flew over Bournemouth I was given the usual "not above 2000" - so went across at 18-1900' on QNH (can't remember what it was). I reviewed the flight on the Bournemouth flight tracker website the next day and was surprised to see my alt being reported at 2200' - so it looked like I was busting airspace.

This begs the question, how does ATC know whether someone is infringing or not via a mode C readout? I could fly along the solent at 1900' QNH, and my alt could be showing 2200' on the radar screen....and hence they could think I am inside their CAS??

[Jumbo Driver]

Yes, Mode C always reports as a FL.

Below Transition, the radar display will normally be set to show the Mode C readout as an altitude, which is derived by a suitable correction being automatically applied for the difference between the appropriate QNH and 1013.25.


JD
:)

[Warped Factor]

The radar processor has an appropriate local QNH input to it.

It then does the arithmetic automatically and shows the Mode C readout as an altitude below the TA and a flight level above. On our system that's by showing altitudes as two digits and flight levels as three.

[Mark1234]

Interesting stuff, hadn't ever considered this angle. I confess it's not at all uncommon for me to nip under airspace out here - I usually leave 100ft headroom, but it's technically legal to (and many do), fly right at the base.

And just in case, I wasn't suggesting going mode A only was a good idea, more curious how it's dealt with - I'm sure there must be a/c out there that have just mode A.. which it seems is in many ways, worse (for the controllers) than not having a transponder at all.

[Fuji Abound]

I wonder how many mode As are left.

On a typical three hour flight this week I reckon around 10% of the returns on TAS appeared to be mode A. Undoubtedly in the nicest way they are a nuisance. You inevitably have no idea whether they are above below or same level and often they seem to be the returns that come and go on TAS - I dont know why this should be but I assume it maybe because the signal generated is "weak" but does seem to be a characteristic of mode A returns. In consequence you are left wondering if it is spurious (although I have yet to have a TAS contact that has definitely proved to be spurious) and it takes longer to visually identify the target.

[LH2]

it's technically legal to (and many do), fly right at the base.

Is it? I cannot remember in which AIP I read that the upper airspace class applies., by way of example, if you had class C between A3000 and FL80, you could technically fly OCAS at FL80, but not at A3000. It might not have been the UK AIP, though, hence why I'm asking. :confused:

I'm sure there must be a/c out there that have just mode A.. which it seems is in many ways, worse (for the controllers) than not having a transponder at all.

Things is, someone who bothers to switch the transponder on is also likely to be on the radio, whereas that's perhaps not so common with non-transponding aircraft. So no I don't think it's worse.

[mm_flynn]

...they seem to be the returns that come and go on TAS - I don't know why this should be but I assume it maybe because the signal generated is "weak" but does seem to be a characteristic of mode A returns.
There are almost no true Mode-A transponders installed. What we call Mode-A is actually a Mode-A/C transponder with the Mode-C frame empty (because Alt has not been selected or no altitude encoder is connected). As such, there should be no difference in the signal strength or quality.

However, I would bet most aircraft that do not use Mode-C are being operated at low level and hence will have intermittent contact with the interrogator, or will have a level of airframe blanking because the transponder antenna is on the bottom of the aircraft and you are looking down at them.

LH2,

I don't have the reference to hand, but believe that the general ICAO principle is that at airspace interfaces the less restrictive class applies. The UK has a special rule about flying exactly at the base (which broadly says you can do it non radio across an airway)

[Fuji Abound]

mm_flynn

Yes, thank you, that makes very good sense. Thinking about it invariably the transient targets I have spotted are invariably very low level.

Despite my earlier comments I guess I would prefer to see a mode "A" contact that nothing at all - although I suppose you never know anything about the others until you spot them or it is too late. :}

I am still fascinated by the targets that follow you! I have had a few that almost exactly parallel your course - sometimes gradually converging, sometimes not. Search as you will it also still fascinates me how difficult they can be to spot and the growing sense you feel that finally you have come across a phantom return. So far I cant say any ever have been phantom returns and it still never ceases to intrigue me when (if) you finally see the other aircraft. I have had a few occasions, including one this week, when you find the aircraft paralleling you was going to exactly the same destination (both outside CAS, non airways) albeit you were over 150 nm away from that destination at the time. This week the other eventually called up and it became obvious who he was, that he was no phantom and why he was paralleling us! We were faster and gradually pulled ahead, but even so I never saw the other aircraft until he eventually landed 15 minutes after us.

[Deeday]

Interesting discussion. To give some context to my original post, when departing to the East, from Redhill, we usually fly underneath Gatwick CTA (base 1500 ft), as close to it as practically possible (terrain elevation gets up to 800 ft, in some places).

The same "anti-Mode C" instructor I was talking about, today did something even odder: we kept the xpdr on Mode A for most of the flight (for the aforesaid reason) then, on our way back, when instructed by Farnborough to squawk 7000 and contact Redhill Tower, he switched it to ALT, just before descending and squeezing below the CTA. I really don't get that. As far as I'm concerned, I think I'm going to keep Mode C on all the time.
Looking back, I'm pretty sure I did all of my qualifying cross-country with just Mode A - an indeed I had a Robin flying right on top of me, tracking the same radial in the opposite direction, not so many hundreds of feet above (not the most welcome sight, on your QXC!)

[steveking]

For your own peace of mind and assuming you rent the same aircraft it may be worth getting a transponder readout check with farnborough to see what accuracy your transponder is giving. It'll give you a bit more of an idea how far to stay underneath.

It'd also be interesting if any ATC guys here would be able to tell us when an aircraft is considered to be infringing, is it when there transponder shows it or do they allow an extra 150ft on the transponder readout for any possible errors.

ie if under a 1500ft airspace would they wait untill the transponder clicked over to 1700ft before the alarm bells ring or is it the moment it goes onto 1600ft.

[Gargleblaster]

"How accurate is your Mode C? " I have no idea ! Why ? It's legally installed and inspected.

Is it a good idea to turn your car's headlights on just before or after sunset ? Yep it is.

So I always sqawk the mode that gives any interested party the most information about my whereabouts.

There are some confusing and contradicting statements in varous countries (including mine's) AIPs / VFGs re. transmitting lesser information, and I've chosen to ignore them. Some revolve around emergencies, and the last thing I'm going to fiddle with in such a situation nis my transponder, unless it's to change the second digit from 0 to 7.

[viva77]

From an ATC perspective an interesting thread and some very knowledgeable replies; I pretty much agree with what’s been said.
Mode C is an aid to your safety as well as others, I can’t see any reason for not having it switched on unless asked to deselect.
My answer to Steve King is that if an aircraft is indicating it is infringing, then we take the appropriate action. If 1 of our aircraft is going to come adjacent to it, we will take avoiding action to try to maintain a safe situation. If however there is no conflict with other traffic, I personally am pragmatic about it and accept the 200’ error, particularly if the mode C readout is not constant. I’ve got more important things in life to do than file paperwork! Do remember though that if you are transiting at the base of controlled airspace, any turbulence which causes a climb will put you into controlled airspace, and a TCAS equipped aircraft could easily get an RA from the climb.
Just for your interest, in my experience, when mode C is wrong it is completely wrong not just a little bit out. I’ve seen aircraft flying at minus 2000 feet, and in the circuit at 30000! :hmm:

[Jumbo Driver]

...Mode C is an aid to your safety as well as others, I can’t see any reason for not having it switched on unless asked to deselect.

I quite agree - in fact, UKAIP ENR 1.6.2 (http://www.nats-uk.ead-it.com/aip/current/enr/EG_ENR_1_6_en.pdf) expects you so to do ...
ENR 1.6.2 — SSR OPERATING PROCEDURES

2.2 Mode A Conspicuity Code

2.2.1 When operating at and above FL 100 pilots shall select Mode A code 7000 and the Mode C pressure-altitude reporting mode of the transponder except:
(a) When receiving a service from an ATS Unit or Air Surveillance and Control System Unit which requires a different setting;
(b) when circumstances require the use of one of the Special Purpose Mode A codes or one of the other specific Mode A conspicuity codes assigned in accordance with the UK SSR Code Assignment Plan as detailed in the table at ENR 1-6-2-5 to ENR 1-6-2-10.2.2.2 When operating a SSR transponder below FL 100 in airspace where the operation of SSR transponders is not mandatory, pilots should select Mode A code 7000 and the Mode C pressure-altitude reporting mode of the transponder except as in sub-paragraphs 2.2.1 (a) and 2.2.1 (b) above.

JD
:)

[Mark1234]

LH2: Just FYI, in the Aus AIP, it definately says where two airspace classes meet / overlap, the less restrictive applies. Sorry, I can't be bothered to look it up!

In my mind it's a moot technicality, as it only matters over a distance of 1ft - how accurate is your altimeter, your flying, and how tall is the fin? Seems sensible to be a bit under, whether thats's 50, 100, or 1000ft..

Interestingly enough, was airborne on the weekend listening to Melb radar trying to get in touch with some chap, who turned out to be on another frequency, (relayed via another a/c). I assume he was transponding as they had a good fix on his location, but when they got hold of him he didn't know what the ident button was, and turned out to be in controlled airspace - by his own admitted altitude, no mode C.

[PD210]

Reading through this thread, replies largely talk about altitude accuracy. However, can anyone shed any light on a transponders accuracy laterally. i.e. putting an aircraft 2nm, 3nm etc from where they believe they are. I have looked through Bendix King web info but could not find anything. Does lateral accuracy depreciate the closer one is to terrain. Can surface structures cause multipath effects similar to GPS antenna locations?

Thanks

[BackPacker]

PD, a transponder (with the exception of extended mode-S I believe) does not emit any lateral data whatsoever.

It just responds to interrogation requests from either radar or TCAS units, and it's the radar or TCAS units which calculate the lateral position (or offset) based on propagation time, direction and such. Of course radar and TCAS will suffer from all sorts of errors, including surface errors but that's really not the transponders problem so you wouldn't find anything about it in the transponders documentation.

I believe PCAS (as in Zaon MRX/XRX or PowerFlarm), also uses signal strength to determine the distance. Reason of course is that PCAS doesn't send out its own interrogation signal so it can't use the timing information from the return signal.

The exception to this, as far as I know, is extended mode-S or whatever it's called. Such a reply can include an ADS-B signal that encodes the exact position. This position is taken from the FMS, who uses some sort of combination of INS and GPS to calculate the position.

[chevvron]

I have known a C172 with a slightly faulty transponder to consistently show its transpoder return about 8 miles displaced from it's primary blip, also many military aircraft have an ECM fit which does this intentionally.

[IO540]

However, can anyone shed any light on a transponders accuracy laterally. i.e. putting an aircraft 2nm, 3nm etc from where they believe they are.

It's down to the radar to place the target laterally.

Radar is not all that accurate - perhaps 1-2nm at 30nm. A lot of computer processing takes place on the data which makes it appear more precise but the basic system is not all that precise. Nowhere near as accurate as GPS for example.

I have known a C172 with a slightly faulty transponder to consistently show its transpoder return about 8 miles displaced from it's primary blip,

How does that work? Could it be caused by the transponder being way too slow to return the data?

[englishal]

I've had an altitude encoder fail and show me at 8,000' when I was at 2000' and had a nervous controller asking me "when are you going to decend?"....Actually that day I forgot to just select Mode A only on the return leg , and to the controller I was now at 11,000' :O

Funnily enough, no one queried the 11,000' all the time I was talking to ATC all the way along the south coast (I did tell them my alt) until on handoff from Southampton after a zone transit, I heard the controller tell another aircraft that there was "IFR traffic at 11,000 just been handed over from Southampton", which is when I remembered, told her and went back to Mode A.

[BackPacker]

How does that work? Could it be caused by the transponder being way too slow to return the data?

Or (but that's maybe less likely) too fast?

I haven't checked but I would assume that in the standard there's a fixed delay specified so that both very slow and very fast transponders will emit their response after the exact same delay. A delay specification "as soon as possible" would be different for each transponder type, particularly if there's some logic in the transponder that might need to pull a GPS location from the FMS, encode it in ADS-B and so on and so forth.

If that delay got misconfigured somehow, you'd either be too fast or too slow, leading to a mispositioning of your blip closer to, or farther away from the radar stations position. Particularly if the SSR return is not correlated with a primary return. Which, as I understand, is the default behaviour for civilian radar. And TCAS of course.

[mad_jock]

I once started showing -4000ft and got told to adopt the callsign submarine, witty chaps at Scottish.

[SNS3Guppy]

I believe a Mode C/TCAS altitude error almost caused a mid air between BA and Korean 10 years ago ..


I experienced a TCAS Resolution Advisory (RA) several days ago, in which I was required to deviate from my flight path in order to avert a collision. We were level, with the autopilot engaged at the time, when the other aircraft descended into our altitude and track. We got a "Descend! Descend!;" I disconnected the autopilot and manually descended. The other aircraft leveled and stopped their descent. We advised air traffic control, then returned to our assigned altitude once clear of the threat. The TCAS worked exactly as advertised.

That event occurred while in positive radar control, while actively talking to controllers, and taking vectors from the controller (meaning the controller has accepted responsibility for separation from other traffic). It occurred at night, in a busy area involving a lot of other TCAS targets; a lot of traffic. I had already ranged mine down to eliminate the high concentration of traffic displayed on my display. We had one TA, or traffic alert, and then less than a minute later, the RA. A resolution advisory is rare; one doesn't need to take an evasive action very often. Without that equipment and that warning, however, I very much doubt any of us would have seen the other aircraft.

Turning your transponder off, especially near busy airspace, is a really poor idea. So is skirting an area of controlled airspace so close that one is in danger of infringing on that airspace, if one isn't participating with the controller.

When I'm flying under IFR, I have no idea much of the time what airspaces I'm passing through. Under IFR, it's largely the same. I don't look, I don't care. I don't have access to that information, really, and I have no idea if I'm close to the bottom of the controlled airspace or going in and out of it. I'm flying a designated routing with a clearance. You might think that you're on the edge of he airspace and thus in good shape, and you may be very, very wrong. Consider several possible scenarios.

I have a traffic alert and respond by making a descent. I'm being operated near the "edge" of the controlled airspace. You're skirting the edge. You've turned off your transponder. I'm taking action to avoid someone who does have a transponder, and the message I get is "Descend! Descend!" If you had your transponder on, I'd have received a different warning, as my transponder TCAS would take into account the actions of your aircraft as well, in establishing the resolution advisory. Instead, your transponder squawking Mode A only, or turned off, my avionics don't "see" you, and we collide, during a legitimate avoidance maneuver.

ATC needs me to descend. They ask me to keep my speed up and give my best rate of descent. You're operating near the "edge" of the airspace, and think that means something to me or ATC. I deploy the speed brakes and pull the power to idle. I'm doing 250 knots and descend into you from behind, with a high rate of descent. I'm going faster than you think, descending into you at several thousand feet a minute. Even if the altitude to which I'm cleared is the bottom of the controlled airspace in that area, if you're right on the "edge," and if your altitude isn't quite correct, it doesn't take much error, especially with a closure rate of 250 knots or greater and several thousand feet per minute vertically, to wind up with a collision.

I'm departing. I'm heavy, so I'm not climbing as fast as the controller might like. I get a turn away from other traffic, and continue my climb. You're flying with your transponder off, or perhaps have it on, but elect to do so on the edge of a busy terminal area, thinking that it's a good idea so long as you're on the "edge." During departure, I'm busy getting cleaned up and configured for the enroute climb, and I don't have a really great view of other traffic, especially at our climb speeds which may be closer to 300 knots, even below 10,000 (minimum safe speed allows higher than 250). I may be on a vector, and I may pass in and out of controlled airspace during that climb. The controller may make a mistake. Why put yourself in the position of being in a traffic conflict in the first place; it's not safe for either one of us.

Those who would skirt airspace as closely as they can remind me somewhat of someone who might walk on the highway as close to the edge of the highway as possible. One might technically have stepped into traffic, but what if traffic drifts across the line, slightly? Doing the same thing in flight with altimetry systems that can be off slightly means that the analogy is a little more like walking along the highway blindfolded; you have no way of knowing exactly where the "edge" is. Give it a buffer. Stay on the sidewalk. Don't go near the edge. Playing near the edge is asking for trouble, whether it's performance, or airspace. I've spent much of my career in operations involving flight near the edge of performance, foreign airspace, and close to obstacles and terrain, and I'm paid in some of my employment to do just that. Are you? Then why go there?

We carry two transponders and encoding systems, and can switch between them. Some of our people will check them both after takeoff and have ATC give them an altitude check, especially before a long flight out of radar contact. We're checking our altitude, our tansponders, and our ability to remain where we say we are. Before going oceanic (class II navigation), we do a position check, verify our radios accuracy and make a comparison with our other nav systems, and do an altitude check. Additionally, there are actually systems in place geographically around Europe and on each side of the ocean which exist just to verify actual altitude vs. the broadcast (mode c) altitude. It's serious stuff.

Why not do the same thin when renting, then? Check your altitude, and get a confirmation from ATC; have a radar controller tell you what the radar controller is seeing.

Bear in mind that if your operating on the "edge" is close to a transition area where aircraft are operating on different altimeter settings, you may have some very significant indicated differences in altitude, yet be at the same height. Not a good idea. If you know the boundary for airspace is at 5,000, for example, why fly at that altitude? 6,000' works, as does 4,000.

If you're going to fly on the ragged edge of an airspace change, why not simply get a clearance in the controlled airspace? If you're not willing to do that (or able, circumstances depending), then you probably ought to give it a wider berth. Avoid the potential trouble.

There are places in the US where encroaching on certain airspace will earn you a fighter escort and a long talk with some very serious-looking people across a cold table in a quiet room with mirrors. Most of the violations of that type of airspace occur when people think they know where they are, or think they're just skirting the airspace, and they're not.

Radar is not all that accurate - perhaps 1-2nm at 30nm. A lot of computer processing takes place on the data which makes it appear more precise but the basic system is not all that precise. Nowhere near as accurate as GPS for example.

Depends on the radar. Radar is accurate enough to enable me to fly an instrument approach to low minimums using only the controllers voice and the radar display that's visible to the controller. Radar tends to be very accurate, especially terminal radar. It's not off by one or two miles; it shows traffic right where the traffic is. Radar separation is based on it being accurate.

Transponder returns are based on the transponder, but unless the transponder is set up using some advanced doppler shift software, the target is right where it appears. Radar information provided to a controller is filtered for local altimeter setting, and controllers can screen out traffic or highlight traffic, but air traffic control radar is quite accurate, and it's very accurate at distances much farther than 30 miles.

"How accurate is your Mode C? " I have no idea ! Why ? It's legally installed and inspected.

Is it a good idea to turn your car's headlights on just before or after sunset ? Yep it is.

So I always sqawk the mode that gives any interested party the most information about my whereabouts.

There are some confusing and contradicting statements in varous countries (including mine's) AIPs / VFGs re. transmitting lesser information, and I've chosen to ignore them. Some revolve around emergencies, and the last thing I'm going to fiddle with in such a situation nis my transponder, unless it's to change the second digit from 0 to 7.

You should know how accurate your mode C is reporting, and should check with ATC occasionally to find out.

You appear to be suggesting that you operate the transponder in Mode C all the time, but there are times and places to operate in Mode A only, or in Standby. When operating in formation, for example, one transponder will speak for the formation, with the others being either in standby or mode A. When on a busy surface area, some airports will require mode C operation for ground surveillance radar, while other places want the transponder in standby or off on the ground. Failure to abide local practice isn't giving extra useful information. It's causing a problem.

You could simply keep pushing the ident button all the time if you really want to stand out to controllers, but you're not helping anyone unless you're asked to squawk ident. Giving the maximum amount of information all the time can be taken to an extreme, and is not helpful.

I once got in a King Air 200 with an FAA examiner for a checkride. He saw the TCAS on board and groaned. As busy as the airspace was, he knew we'd be getting traffic alerts constantly. In such cases the TA's could actually cause a safety issue and at a minimum an unwanted distraction, and trying to avoid an aircraft being called out as a traffic alert could cause a collision hazard with another airplane. In that area, see and avoid was absolutely essential, and I kept my speed down and my head up until we were well clear of that airspace, accordingly.

However, can anyone shed any light on a transponders accuracy laterally. i.e. putting an aircraft 2nm, 3nm etc from where they believe they are.

Mode A and C transponder returns don't carry any information regarding lateral position. Radar determines where you are. The transponder simply makes you a little more visible and tells the controller who you are, and how high you are.

The same "anti-Mode C" instructor I was talking about, today did something even odder: we kept the xpdr on Mode A for most of the flight (for the aforesaid reason) then, on our way back, when instructed by Farnborough to squawk 7000 and contact Redhill Tower, he switched it to ALT, just before descending and squeezing below the CTA. I really don't get that.

Find a different instructor, preferably one who knows what he's doing.

[AdamFrisch]

Don't most of them report standard pressure altitudes only, ie FL's -29.92/1013mb? Meaning they can of course be off by quite a lot in a low pressure/high pressure scenario.

I'm not sure weather the controllers base their height for infringements on primary contacts or the Alt mode return, but I'm guessing the xpndr return.

[steveking]

I think ATC software automatically adjust FL into QNH where needed.

[Whiskey Kilo Wanderer]

My Mode C is usually within 100’ of the altimeter reading. However the altimeter can be out as well, due to instrument and static port error (the latter affects the encoder as well in my case). The GPS altitude readings seem to agree and to be the most accurate in absolute terms.

If I’m ducking under low (1,500’ altitude) stubs like Gatwick and Stansted, I’ll usually ask for a ‘Mode Charlie Check’ from Farnborough or whoever. That way they know that I know that they know….

The whole ‘turn off the Mode C’ approach may well be looked upon most unfavourably by NATS / CAA. If you get a chance to go to one of the NATS Infringement Presentations, it’s time well spent. They cover the effects of unknown aircraft infringing controlled airspace. One interesting presentation covered an infringement of Stansted, by someone who thought he was landing somewhere else. It caused absolute chaos and knock-on effects to the airlines for the following twelve hours. Because the pilot had made a genuine mistake, he was let off with an invitation to ‘get a bit more training’. The word from NATS that anyone thinking they have infringed and trying to escape by running away low level or knocking off the Mode C, is liable to get the proverbial book thrown at them.

Safe Flying,
Richard W.

[Jumbo Driver]

All airborne Mode C encodings are always referenced to 1013. SSR altitude information must be referenced to the same datum to ensure correct TCAS operation.

Information can be screen displayed to controllers as referenced to QNH but this is only selectable on the ground for display purposes.


JD
:)

[BackPacker]

I'm not sure weather the controllers base their height for infringements on primary contacts or the Alt mode return, but I'm guessing the xpndr return.

Primary radar, unless you've got very sophisticated, military-style multi-lobe radar, doesn't give any altitude information, just lateral information (position).

The altitude is solely based on mode-C returns. No mode-C, no altitude, simple. And I believe in the UK any aircraft squawking mode A only, is assumed to be below the base of whatever controlled airspace is concerned.

All airborne Mode C encodings are always referenced to 1013. SSR altitude information must be referenced to the same datum to ensure correct TCAS operation.

True. That's why you will never have to put the altimeter setting in your transponder. The datum is always 1013.2.

However the controller has the ability to enter the local QNH (or QFE I suppose) into the radar scope so that he can watch the altitude (or height) of aircraft that operate below the Transition Layer.

[eglnyt]

Modern radar processing systems will automatically convert the Mode C below the transition using the appropriate value for QNH. Some can cope with multiple QNH values and I have seen aircraft apparently jump several hundred feet when they pass from one QNH region to another when the QNH values were significantly different because of a fast moving low pressure region.

Transponders have a delay deliberately introduced into the processing and this is set up during commissioning and maintenance so that all transponders have the same delay between receive and transmit. If the delay is wrong the reply will appear to come from the wrong place but generally even at the limits it's still not going to move the position very far.

I've seen a number of cases similar to that described by Chevron. They happen when the non Mode S transponder inadvertently responds to Mode S interrogations. As the Mode S interrogation preceeds the Mode A interrogation the result is that the Mode A reply is sent before it should be and appears to be much closer to the radar head than it is. The transponder will normally also respond to the correct interrogation so two replies are received.

How that appears to the controller will depend on the processing applied. He may see one target in the wrong place or two targets one of which is in the wrong place. The primary target will be in the right place if the controller has primary selected.

[A and C]

This all depends on the type of encoder you have fitted, the older annolog (grey code) encoders report in 100ft increments but the newer digital types can report in 25ft increments.

This is for TCAS & ADS-B traffic avoidance technical reasons but I doubt if this shows up on the ATC radar screens however I am sure that the data is used in the ATC computor system to refine the collision avoidance sytems.

The word idiot would likely spring to mind in the same sentence as someone who turns off the mode C if they have it fitted. A lot of light aircraft are now fitted with transponder based traffic systems, you may not be able to see them but with the transponder switched to mode C they can see you.

All the RAF Grobb 115 aircraft will be fitted with a traffic system within the year if you keep the mode C on that is another 95 aircraft you will be unlikely to hit !

[lucaberta]

A very interesting topic, let me add my 2 europence to the discussion, quoting some previous messages and adding a comment.

When I'm flying under IFR, I have no idea much of the time what airspaces I'm passing through. Under IFR, it's largely the same. I don't look, I don't care.trust you don't do this when you're flying in the USA! Over there it's quite normal to have a mix of VFR and IFR traffic at altitude (typically Class Echo airspace), but in most cases ATC will be able to tell you the reported altitude if the VFR traffic is not in radio contact (they would say "altitude unverified" in that case).

The fact that under IFR you should never have any VFR traffic nearby is very much a European thing, as if all the airspace is always class Alpha. Well, in some cases it really is Alpha, like in the huge TMAs here in Italy...

Mode C transponders have a 100 feet resolution, but if you have a Mode-S transponder, its resolution is 25 feet. Since introduction of Mode-S we often see airliners crossing a whole FIR at FL301 (30050 feet)...I believe that the 25' resolution is not mandatory for Mode S, and I am sure that a lot of Mode S installations on GA aircraft still use the old blind encoder feeding their gray code altitude via the 12 bits parallel wires, and that only supports 100' resolution.

All the newer encoders support serial data out, and the newer transponder often have an integrated blind encoder supporting 25' resolution.

Now, my comment on using Mode A alone, without any Mode C reporting. Are we really seriously still doing this today? This is madness! :=

As far as I know, if a Mode A only transponder is interrogated by TCAS, the lack of altitude reply via Mode C will give the worse case scenario to the TCAS interrogator, and TCAS will put the intruding aircraft at the same altitude as the interrogating aircraft. There is no way to escape such an RA other than a lateral deviation, though maybe you are thoudsands of feet separated vertically. You just don't know about that since Mode C is off.

In my opinion, in case of a known Mode C failure before the flight, better avoid flying altogether, and if it happens in the air while in contact with ATC, they will tell you quite soon like some have told us in this thread.

But knowingly using only Mode A, to me is a no-no. Why not just turn off the whole transponder altogether?

Ciao, Luca

[Deeday]

This is a bit astonishing.

Re-reading this thread that I started, only now do I realise that Mode-C sends back flight levels that are then converted into altitudes by the radar software - and that my instructor did not have a clue about this fact! :ugh:

When I was advised not to use Mode-C under a CTA, I was a student; the transponder was a steam-gauge one and the reason given that it was not accurate made some sense to me, at the time.

After getting my license, I got checked out on another aircraft, with a Garmin digital transponder that shows you the offset between actual and pressure altitude, and the advice became "Don't use Mode-C as it's set on 1013 mb and cannot be changed, which gives wrong altitudes most of the time", leaving me with the unanswered question as to why they would build Mode-C transponders that are useless most of the time... :ugh: :ugh: :ugh:

The advice "Ask your instructor, instead of relying on an internet forum" doesn't always apply... :rolleyes:

[A and C]

You are quite correct about the 25 ft resolution being required for Mode S but as the encoders cost the same as the old 100 ft resolution encoders they are usualy used mostly because they are much less trouble to wire.

The digital encoder requires 4 wires total, the older Grey code units need about ten wires.

[ShyTorque]

Deeday, I'd be tempted to ask your instructor what his idea of how a transponder works was based on. Doesn't sound like it was based on a real working knowledge.

As a TCAS user in Class G airspace, please use mode C whenever your aircraft has it fitted. We can use TCAS to to know where you are and we can often adjust our flightpath well before either of us gains visual contact; to make life safer for both of us.

Btw, our transponder has a readout of the altitude being sent. I use that to crosscheck against QNH set and altitude; so I know that it is correct.

[Deeday]

Deeday, I'd be tempted to ask your instructor what his idea of how a transponder works was based on.Well, you would have to ask her then http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/smile.gif

As far as I'm concerned, everything is clear now. You bet that I'll switch Mode-C on :ok:

[lucaberta]

Deeday, I am astonished to read what you said about your instructor. :mad:

Good news, you've finally understood why it's good to always have a transponder turned on to Mode C. And what helps us sometimes is exactly the reason that your instructor was blaming, and that is the fact that *every* transponder uses 1013.25 as their reference. This is fundamental for TCAS, as what matters is a relative altitude and not an absolute one.

Really surprised someone could teach with such a gaping hole in her aircraft knowledge... :=

Ciao, Luca

[IO540]

A lot of instructors are not all that clued-up. My main PPL instructor said a VOR (not a DME) stops working if too many planes tune into it, because the transmitter sends out a special signal which sets up the TO/FROM flags.

The full explanation of how the flags are driven is actually fairly complicated (to a non-electronics person) so this kind of crap is not implausible unless you know better.

What gets up my nose however is how many people are simply non-transponding. I flew for 2.5 hours today, in the south east, and got a traffic service about 75% of the time. Not one of the 30-50 or so contacts was transponding.

The cloudbase was generally 800-1500ft AGL and the tops were 3000-4000ft. I was VMC on top and worked around the LTMA to stay there, because there was quite a bit of ice in the cloud below (temp -2C, against +3C above under the blue skies).

I don't think they were gliders, either...

[lucaberta]

Where they microlights, you reckon? What are the rules for the use of transponders by microlights in the UK?

Here in Italy I encourage people with a microlight to have a transponder with Mode C on board, and to keep it on ALT, always, even if they are not in radio contact, just because of the added bonus of being seen by TCAS in case of straying into CAS by mistake.

I doubt that weight-shifting trikes would have a transponder, though. And gliders with that weather, difficult, unless you were just above a field with a winch, but they should be published.

Ciao, Luca

[IO540]

What I do find is that about 99% (no kidding) of the "altitude unknown" contacts reported to me by the radar controller and which I manage to get visual with, turn out to be flying very low.

Obviously, my first reaction to a report is to look around my own level, plus or minus say 30 degrees, and then only if somewhat bored I start scanning the ground below... or sky above.

How low is hard to judge and "low" traffic appears a lot lower than it is but my guess is below 1000ft AGL and that does tend to correspond to the "ultralight" community. But they are powered and can power a transponder.

I have never seen an "altitude unknown" contact at my level or higher. Make of that what you want. My take on it is that most of the "civil liberties" crowd which insists on avoiding transponders flies at low levels. I suppose this is better than having them fly at 2000ft plus or in IMC; one has to be grateful for small mercies :) It also means that one can dramatically reduce one's chances of a mid-air, and the stats support this.

[soaringhigh650]

What gets up my nose however is how many people are simply non-transponding. I flew for 2.5 hours today, in the south east, and got a traffic service about 75% of the time. Not one of the 30-50 or so contacts was transponding.

Non-transponding traffic was definitely one of my most annoying encounters when I was flying in the UK. I spent some time looking for things which may or may not have been in conflict with me.

[SNS3Guppy]

My airline pilot friends tell me that even a 747 at max weight climbs at 250 below 10,000 ft.

Your airline pilot friend is wrong.

Our max gross takeoff weight is 833,000 lbs.

I just calculated our takeoff performance using present location present conditions, for a max gross takeoff, and came up with a V1 speed of 160 knots, a rotation speed of 179, and a V2 (takeoff safety speed) of 188 knots. Our climb speed once flaps are up will be 100 knots over V2, which is 288 knots in the climb. We would ask for a high speed climb, and get it (as that's our minimum speed on the climb-out.

According to these calculations, we couldn't raise flaps above our "1" setting if we intended to restrict speed to 250 knots, and we're not going to do that. That leaves flaps and leading edge devices out for the climb and we don't climb like that or predicate climb performance on that.

If minimum safe climb speed is 300kts below 10,000 ft, what's Vref?

Sounds like a long runway is needed whatever it is!

We use Vref for landing, not for takeoff. The takeoff distance on a 12,000' runway, using a reduced takeoff power setting, shows a stop margin of 2,900 feet today. That's assuming maximum reversing and a rejected takeoff no later than V1.

If we did need to dump fuel to come back and land, my theoretical flight would take 51 minutes of dump time down to our max landing weight of 630,000 lbs, a Vref speed of 152 knots, and a landing distance of 8755' (calculated with minimum autobrake setting). That also adds up to a 237 knot minimum speed with flaps up (we use 20 over that, so 257 will be our minimum speed until configuring for landing).

Just for kicks, I ran the same approach to the same runway under the same conditions, and added a trailing edge flap assymetry. Our landing distance increased to 10,844' in minimum autobrakes. Approach speeds bump up 20 knots.

If we leave the flaps alone and let them function normally, but find we're unable to dump and have to land at takeoff weight a fire on board, for example), our Vref speed becomes 179 knots, our target speed on approach is 192 knots, our landing distance in minimum autobrakes becomes 11,827'.

These are actual numbers using the same performance program we use on the line, just installed on my laptop, from the warm comfort of the hotel room. These use the same conditions in which we landed a few hours ago, in the same airplane.

The short answer to your question is that on takeoff, we always see the red lights at the other end of the runway.

What gets up my nose however is how many people are simply non-transponding.

I don't know about the UK, but airplanes without electrical systems, radios, or transponders are still common in the USA. While I encourage people who have transponders to use them, of course, I encourage everyone to use eyes to look for traffic and rely on radios or gadgets to do that for them.

The life you save, after all, may be your own.

[Rod1]

Transponders in micros

Unfortunately we have a very restrictive micro definition compared with most of Europe and else ware. We have a concept of a max empty weight, which is more rigorous than most of the three axis designs were intended to meet. The result is that many micros (the CT for example) are within 1lb of the max allowed empty weight. If you add more than 1lb of installed weight to such a design it becomes illegal and is reduced to the level of a garden ornament. This is preventing many micro owners from installing transponders. Many of the aircraft in this category are capable of cursing at 120kn and have a r of c in excess of 1200 fpm. Most of the pilots are people who grew up flying PA28’s etc and my experience is they fly at regular GA levels when conditions are appropriate.

Many UK owned gliders have no approved transponder installation. As the manufacturers of many of these machines do not exist it is virtually impossible to install a transponder, even if there is sufficient batt power.

Both of the above issues were brought up at the Mode S meetings, so the CAA/EASA has had lots of time to change things.
When I was researching my collision avoidance article for Flyer we came to the conclusion that about 50% of the things your average PPL are likely to hit are transponder equipped. I would not expect this to have changed greatly since.

Rod1

[John R81]

Fascinating reading!

It never occurred to me to switch-off Mode C (other than as part of a "flight") until I was told by a pilot (in my mchine) that he had been asked to do so over London (H4) at 1200ft by a controller terrified by his "sudden descent" on screen. Certainly livened up the day for that controller! Fault traced eventually to corrosion in the terminal connection to the transponder arial. In the process, I swapped to a Garmin 330 Mode S.

As to accuracy of the transponder, when the QNH is close to 1013 I can read the flight level against altimeter; I believe the 25' accuracy statement! I can in any case compare FL on the transponder with the altimeter, and then with the altitude shown on my Flymap 7. Allowing for pressure differences, not a bad check. I do this regularly to watch for drift in the altimeter.

Non-transponding aircraft?

Worst "offence" I saw was when heading into Redhill from the East under Gatwick's 1500ft limit at 1200ft, just North of the railway line. A Cessna came accross my path from South South East but curving around to eventually head North East over the town of Godstone at what I guess to have been no more than 200ft. At first I called it in on the radio, fearing he was in trouble, but eventually he climbed away towards Biggin. Caused excitement for ATC as this guy was invisible to their screens. Wonder why he had the transponder off!

Worst "non-offence" situation for me - gliders and microlites. No transponder might be legal and understandable (weight point made above), but you guys are much harder to see and as I am helicopter (tending to operate about 1,000 to 1,200 ft AGL) certainly we are operating in the same space. My only suggestion is to ask if you kit-out with a radio and talk to the LARS service? At least then I can hear your reports to get an idea where you are (and you can hear mine, too). Mostly those that I meet are not talking with the LARS unit.

[BackPacker]

As to accuracy of the transponder, when the QNH is close to 1013 I can read the flight level against altimeter; I believe the 25' accuracy statement!

My usual aerobatics height band is between 3000' altitude on the local QNH, and FL55. I leave the altimeter on QNH, and read the FL from the transponder. Works fine. You just have to keep in mind that if the QNH is < 1013, the height band available might be (significantly) less than 2500'.

My only suggestion is to ask if you kit-out with a radio and talk to the LARS service?

A lot of gliders have a radio but do not have their FRTOL. So they're only allowed to operate on specific glider frequencies. That rules out getting a LARS.

[Mark1234]

ATC needs me to descend. They ask me to keep my speed up and give my best rate of descent. You're operating near the "edge" of the airspace, and think that means something to me or ATC. I deploy the speed brakes and pull the power to idle. I'm doing 250 knots and descend into you from behind, with a high rate of descent. I'm going faster than you think, descending into you at several thousand feet a minute. Even if the altitude to which I'm cleared is the bottom of the controlled airspace in that area, if you're right on the "edge," and if your altitude isn't quite correct, it doesn't take much error, especially with a closure rate of 250 knots or greater and several thousand feet per minute vertically, to wind up with a collision.

I'm departing. I'm heavy, so I'm not climbing as fast as the controller might like. I get a turn away from other traffic, and continue my climb. You're flying with your transponder off, or perhaps have it on, but elect to do so on the edge of a busy terminal area, thinking that it's a good idea so long as you're on the "edge." During departure, I'm busy getting cleaned up and configured for the enroute climb, and I don't have a really great view of other traffic, especially at our climb speeds which may be closer to 300 knots, even below 10,000 (minimum safe speed allows higher than 250). I may be on a vector, and I may pass in and out of controlled airspace during that climb. The controller may make a mistake. Why put yourself in the position of being in a traffic conflict in the first place; it's not safe for either one of us.


Accepting there are undoubtedly reigonal differences, in Australia (and allegedly the UK, though I can't find a reference), if you are in controlled airspace, a 500ft protective buffer to the base is maintained; AC 2.5.1 Section 8.5 refers. I cite these, only because they are jurisdictions in which I have flown. In these places, you do not fly 'in and out of controlled airspace'. Your SIDs and STARs are predicated upon you being able to achieve certain performance in order to comply, and to maintain the protection of that airspace. You are either inside and get the benefit of 'protection', or outside, and play the way the rest of us have to, controlled classes of airspace are there for the simple reason that high speed, heavy jets generally require such protection. I freely admit I know jack about how the US does it.


Those who would skirt airspace as closely as they can remind me somewhat of someone who might walk on the highway as close to the edge of the highway as possible. One might technically have stepped into traffic, but what if traffic drifts across the line, slightly? Doing the same thing in flight with altimetry systems that can be off slightly means that the analogy is a little more like walking along the highway blindfolded; you have no way of knowing exactly where the "edge" is. Give it a buffer. Stay on the sidewalk. Don't go near the edge. Playing near the edge is asking for trouble, whether it's performance, or airspace. I've spent much of my career in operations involving flight near the edge of performance, foreign airspace, and close to obstacles and terrain, and I'm paid in some of my employment to do just that. Are you? Then why go there?


Again, I suspect you have a reigonally-biassed view - In many parts of the world, being allowed to play in controlled airspace is unlikely at best; furthermore, in my part of the world the bottom of your controlled, protected areas is remarkably close to the ground - leaving us mere mortals quite a narrow zone in which to fly. When the controlled airspace is 1000ft AGL, and I have little hope of a clearance, you can bet your last dollar I'm going to have my fin right up against the bottom of it. You might equally well ask why the professionals need fly so close to the base, and the wild, uncontrolled chaos that reigns beyond..

[John R81]

A lot of gliders have a radio but do not have their FRTOL. So they're only allowed to operate on specific glider frequencies. That rules out getting a LARS. 31st Jan 2011 13:14


Surely that cannot be a reason pilots actually cite- "Better to risk a collision than to sit the exam for FRTOL"???? That seems to me to be as sensible as turning off Mode C (see all of above) - I know that I can, it's just that it would be so stupid at most times.

Still, we live in the real world and at present in the UK South the least visible of aircraft are the ones with no transponder and not on frequency. Seems from your post microlite and glider pilots simply accept that enhanced risk. LOOK OUT is the only option.

[IO540]

How often is the empty weight checked and by who?

It cannot be checked more often than at the Annual, so..... you remove the transponder for the Annual, possibly leaving behind just the antenna. The antenna can be installed (concealed) inside the hull if the hull is composite.

The empty weight is meaningless in safety terms because all the pilot has to do is eat a few more burgers and ... and GA pilots are hardly a health conscious lot.

[BackPacker]

Surely that cannot be a reason pilots actually cite- "Better to risk a collision than to sit the exam for FRTOL"????

I doubt there are a lot of gliders that reason this way. In fact, I doubt that there are a lot of glider pilots that know that there are ATSOCAS services available to them anyway. In general, I think their belief is that you only need the FRTOL if you want to pass through controlled airspace - which they rarely do.

Furthermore, the reality is that a lot of gliders *will* use the radio during their soaring flights. But they're on one of those dedicated glider frequencies and they will use it to keep each other updated on where the lift is, what altitude they're circling and so forth. Even if they wanted to (and were allowed to) talk to a LARS unit, they would need to switch to the LARS frequency and would be missing out important, possibly vital information from their fellow gliders. And no, a typical glider does not have the kit to monitor two frequencies at once.

All that, of course, apart from the distraction that this may cause. Which might be a reason for a glider pilot to switch off his radio altogether and rely on the Mk. 1 eyeball alone.

I'm not defending the glider community here. But I am dipping my toe into their waters (with a bit of luck and planning I should be able to get my GPL this summer) and I find that gliders have a completely different attitude towards radio, transponders and a few other things that power pilots simply take for granted.

[IO540]

I find that gliders have a completely different attitude towards radio, transponders and a few other things that power pilots simply take for granted.

I am sure that is true.

The problem is that a lot of power pilots also have a completely different attitude towards radio, transponders and a few other things that other power pilots simply take for granted ;)

[pulse1]

Over the last two years I have asked several ATC units from OCAS to check our Mode C and the result has been OK. However, during a recent transit of Solent CTA at 3000', we were asked to switch off our transponder because they said it was reading 200' low. We later asked Bournemouth and Bristol Radars to check it and they said it was within limits. Calculations based on the displayed PA and the QNH confirm that it is slightly low but within limits.

As a Permit aircraft it doesn't usually go anywhere near expensive avionics facilities, how else can one verify the accuracy of Mode C?

[soaringhigh650]

Also, ever wondered why you need to state your altitude when you check-in on a new frequency? It's because the controller in the next sector needs to verify your mode-C reading.

So you'll find out pretty quickly if your mode-C is inaccurate!

[John R81]

BackPacker - always good to get a little insight into how other people think / reason.

Me, I would just carry 2 radios.
Come to think of it - I do have two radios in the cab. Overcautious. I should clearly live a little - take more radio risk!

[ShyTorque]

During departure, I'm busy getting cleaned up and configured for the enroute climb, and I don't have a really great view of other traffic, especially at our climb speeds which may be closer to 300 knots, even below 10,000 (minimum safe speed allows higher than 250). I may be on a vector, and I may pass in and out of controlled airspace during that climb.

SNS3Guppy,

Under what permission do you do that?

[Rod1]

IO I did not see a smiley on this post, I hope that was an oversight!

“How often is the empty weight checked and by who?”

To get it installed requires paperwork, which includes the weight of the installation plus the old empty weight. It would also get checked after any accident or incident if the insurance or the CAA thought there was a problem.

”It cannot be checked more often than at the Annual, so..... you remove the transponder for the Annual, possibly leaving behind just the antenna. The antenna can be installed (concealed) inside the hull if the hull is composite.”

So, you are helpfully suggesting that people fly illegally and with a set-up, which would not work? Most of these machines will be composite – carbon fibre – and no you cannot hide the antenna in the fuselage cuz it wont work, would be illegal and no avionic shop would do it (we are talking factory built £80k+ machines)

”The empty weight is meaningless in safety terms because all the pilot has to do is eat a few more burgers and ... and GA pilots are hardly a health conscious lot.”

I and many others have tried to get the CAA to relax the rules to allow the installation to be excluded from the empty weight calculation. Many parts of the world have similar exceptions, but the CAA would not budge. Perhaps it would have been a better use of my time posting ridicules bits of advice on an aviation forum…:p

It is rules like the empty weight rule that make the VLA solution, which has no such silly rule, a better option.

Rod1

[lucaberta]

SNS3Guppy,

Under what permission do you do that?
that is a good question. Over here in Europe there are no strict regulations that force 250kts below 10,000', as far as I know (at least here in Italy), but in the USA no controller can issue deviation from an FAR which specifically says that every aircraft *must* be flying slower than 250kts below 10,000'.

I would be curious to know if you really keep the leading edge flaps out until 10,000' during climb when you're in the USA.

Ciao, Luca

[Mark1234]

There's another thing that goes against gliders and transponders, multiple radios and the like - POWER. Specifically, everything must run off batteries, and when flights are frequently in the reigon of 5-10hrs, that can be a real issue.

I also rather doubt the practicality of working under ATSOCAS for gliders - they don't fly in straight lines, maintain altitude, speed, or anything like that for very long; if you were to tell someone every time you changed height/speed/direction, you'd never stop transmitting.. They also tend to want to look out the window, and enjoy flying, not play at airline pilots(!) Yes, I have a foot in both power and gliding camps..

@ShyTorque - In Aus it's pretty common to hear the 'heavies' ask for (and receive) "high speed climb", or "cancel speed restriction below 10,000" - As I understand it, the ATSU has the authority to allow as they see fit. Passing in and out of controlled airspace - er - goodness knows!

[ShyTorque]

Passing in and out of controlled airspace - er - goodness knows!

The "out of controlled airspace" is the part I was more interested in.

[John R81]

Mark

Messing around in a helicopter might give rise to as many changes in heading / height. Don't make so many radio calls that a battery operated hand-held would run out of juice.

Just the occasional call would let everyone on station have an idea where the glider is at that time.

[mm_flynn]

that is a good question. Over here in Europe there are no strict regulations that force 250kts below 10,000', as far as I know (at least here in Italy), but in the USA no controller can issue deviation from an FAR which specifically says that every aircraft *must* be flying slower than 250kts below 10,000'.

I would be curious to know if you really keep the leading edge flaps out until 10,000' during climb when you're in the USA.

Ciao, Luca

Guppy's comments are directly relevant to the US where there is a hard 250 knot limit, UNLESS MINIMUM SAFE AIRSPEED dictates otherwise. There is then a list of aircraft to which this is likely to apply. Which I believe includes Guppy's 747. In the case of the FAA they do not define minimum safe airspeed as the minimum you can climb dirty, it is a clean climb configuration with sensible safety margin (can't seem to find the exact detail )

[SNS3Guppy]

However it is definitely possible to climb out and comply with the 250kt rule. Yes it may require flaps but it is perfectly possible and safe.

Economic; probably not, but since when has economy been the yardstick of "minimum safe airspeed"?

I'm not saying low level climbs at >250kt aren't done, just that it sounds like an airline accountant's interpretation of the rules.

No, it's not. We don't climb with gear and flaps out for a good reason; our climb performance, ability to meet departure procedure and climb gradient requirements, etc, is met in a clean configuration. We don't clean up because it's cheapest. We clean up because once we've met our immediate segment and obstacle climb gradient criteria, we clean up to meet our best rate criteria.

Flying a large airplane isn't the same as flying a Cessna 172 in this respect. Everything is numbers-driven. We have minimum performance criteria which requires certain operational practices, and it's part of the certification (and thus use) of the aircraft. We're also constrained by operational practices dictated through various policies, both governmental, industry, and standardized practices that have been evaluated to meet governmental and international guidelines.

Bear in mind that we operate everywhere but Antarctica, regularly. We're fully ICAO compliant, and we operate in a lot of places where terrain, noise abatement, airspace, and other requirements dictate that we stick to our climb profiles. After a standard departure, we clean up at 3,000', accelerate to V2 + 100, which is our minimum safe speed clean, and climb to 10,000, where we normally accelerate to 320 to 340 knots indicated for the initial climb.


Under what permission do you do that?

As a function of our IFR clearance, and our operations specifications issued to us by the government agency that oversees our operation...which includes operation in uncontrolled airspace. We don't ask what type of airspace we're in during a flight under IFR. We don't particularly care if it's class A, B, C, D, E, G, etc.

If you're asking about the permission for making a high speed climb, the requirement for 250 knots below 10,000, where applicable, applies only if the minimum safe speed of the aircraft is below 250 knots. We don't need permission. We "ask" as a courtesy, for a high speed climb, but also often simply inform ATC about our climb speed. ATC expects it. The one exception will be times when a departure procedure may require a slower speed and a configuration change may be delayed because until those requirements are met, the SID or DP establishes the speed. At that point, our primary concern is meeting climb gradients and crossing restrictions.

Accordingly, we also utilize turn procedures for engine-out situations on takeoff, at many locations, which are different than what are published for themasses. Our turn procedures are issued specifically to us, and may very well take us into uncontrolled airspace. If our turn procedure is a left turn to 180 degrees and our performance calculations give a level off altitude of 800', then we're climb to 800', leveling off, turning to 180 degrees,and advising ATC that we'll get back to them. ATC doesn't have this procedure; we advise them of what we're doing, make appropriate requests or statements, and handle the problem.

As another aside, part of our protocol when returning to land with certain malfunctions is to inhibit the TCAS RA functions by going to a TA mode only; we'll get traffic alerts, but not a resolution advisory. Our changes in performance or capability dictate that we don't presume to act on that traffic information. What that does is enables our TCAS to interface with other TCAS to require the other aircraft to come up with a resolution that doesn't need us to deviate. Such situations are specific, but tied to the idea where that we have performance limitations and procedures to abide, and before others in an uncontrolled situation elect to take full advantage of the extreme limits of their uncontrolled airspace (and let their PCAS do the traffic scan for them), they should be aware that three quarters of a million pounds of aluminum might be getting set to come through their windscreen. Plan accordingly.

Again, I suspect you have a reigonally-biassed view - In many parts of the world, being allowed to play in controlled airspace is unlikely at best; furthermore, in my part of the world the bottom of your controlled, protected areas is remarkably close to the ground - leaving us mere mortals quite a narrow zone in which to fly. When the controlled airspace is 1000ft AGL, and I have little hope of a clearance, you can bet your last dollar I'm going to have my fin right up against the bottom of it. You might equally well ask why the professionals need fly so close to the base, and the wild, uncontrolled chaos that reigns beyond..

I'm not sure how much of a regional bias I could possibly have. At the moment I'm typing from a hotel room in Korea. A little later I'll be in China. I was recently in the middle east, and Europe just before that. I operate about as globally as you can possibly get, so which region is it that influences me? Did you form an impression off what you thought you knew based on the little bit of information under my screen name? I've lived and flown and worked all over the world, and still do. Where my mail goes doesn't necessarily influence or color my "regional bias."

I've spent a lot of hours and years flying close to the surface in utility operations (ag, firefighting, animal survey, back country charter law enforcement, etc), much of it VFR. Much of that VFR flying, in fact transitions in and out of controlled and uncontrolled airspace without any regard to which is which, due to operational necessity. One thing I always do, however, is maintain a high awareness of where traffic conflicts can occur.

A buffer between the lower surface of a busy controlled terminal area and the upper limit of uncontrolled airspace is a potential conflict area. Giving it a buffer is in your best interest.

Sure, as a pedestrian you have the right of way over motor traffic. Betting your life on exercising that right of way against a road train or semi-truck is stupid at best, and extremely dangerous. Betting your uncontrolled rights in your spam can against the sanctity of the lower limits of the local controlled airspace, is likewise not a smart move on your part. Insisting on "sticking your fin" out of the uncontrolled tank may not be your wisest act.



How often is the empty weight checked and by who?

As often as one wants. The question is why check it more often than necessary, or more often than it's changed? Re-weighing is generally done at every annual inspection or every three years, depending on the aircraft and maintenance program under which it's operated.

By whom? Authorized personnel.

If equipment is removed, the weight and balance paperwork must be amended before the aircraft is released for service, again.

It cannot be checked more often than at the Annual, so..... you remove the transponder for the Annual, possibly leaving behind just the antenna. The antenna can be installed (concealed) inside the hull if the hull is composite.

Sure the weight can be checked more often than the annual. However, if a change is made in the weight and balance of the aircraft, the weight and balance paperwork must reflect this change, and should be amended by the person removing the equipment.

The empty weight is meaningless in safety terms because all the pilot has to do is eat a few more burgers and ... and GA pilots are hardly a health conscious lot.

The weight of the pilot is meaningless and irrelevant to the topic of empty weight, is it not? Pilot health consciousness has no bearing on empty weight of the airplane

[ShyTorque]

SNS3Guppy, The reason I asked was because under UK aviation law, you need a written permission to go above 250 kts in Class G airspace. I don't think ATC can give ad hoc permission on the day; Class G doesn't "belong" to ATC.

If you don't have it then don't be surprised if folk complain. I know one corporate operator who was warned off and threatened with further action for flying over 250kts. The evidence was his mode S transponder readout.

[Roffa]

ST,

SNS3Guppy, The reason I asked was because under UK aviation law, you need a written permission to go above 250 kts in Class G airspace. I don't think ATC can give ad hoc permission on the day; Class G doesn't "belong" to ATC.

Correct.

UK MATS Pt 1...

In Class E, F and G airspace, conflicting traffic may not be known to ATC and so it is necessary for all flights to make use of the 'see and avoid' principle. In order for this to operate effectively, controllers shall not authorise a relaxation of the airspace speed limit.

[mm_flynn]

There aren't likely to be any non-emergency, non- military heavily loaded heavies operating a climb profile in the UK outside controlled airspace, however, his point is similar to points UK ATCOs have previously made about why cutting a couple hundred feet into the bottom of LHRs departure airspace is a problem (which is not because aircraft are often scrapping the bottom, but sometimes they are - plus the obvious issue of 'it is against the rules')

[mad_jock]

There aren't likely to be any non-emergency, non- military heavily loaded heavies operating a climb profile in the UK outside controlled airspace

The only one that springs to mind would be Filton possibly but a turn after departure would take them quite quickly into Cardiff airspace.

[Roffa]

mm_flynn,

There aren't likely to be any non-emergency, non- military heavily loaded heavies operating a climb profile in the UK outside controlled airspace, however, his point is similar to points UK ATCOs have previously made about why cutting a couple hundred feet into the bottom of LHRs departure airspace is a problem (which is not because aircraft are often scrapping the bottom, but sometimes they are - plus the obvious issue of 'it is against the rules')

The only departure that these days is likely to be close to the bottom of CAS on departure from LHR (barring unusual circumstances) is the A343 or A346. Everything else goes up at a reasonable rate.

Around LHR it is 09L/R arrivals that are far more likely to be embarrassed by anyone marginally inside CAS as, out to the west where the CAS base is 2,500ft or 3,500ft, the LHR arrivals are routinely only 500ft above said base altitude.

[SNS3Guppy]

If you don't have it then don't be surprised if folk complain. I know one corporate operator who was warned off and threatened with further action for flying over 250kts.

I very much doubt he was flying an aircraft that had a minimum safe clean speed in excess of 250 knots. If he was warned for flying above that speed, it's because he had the capability of flying at a lesser speed, and elected not to do so.

[ShyTorque]

The UK regulations don't make that distinction. If you need to do it, you still need written permission, or you are still breaking the law.

[soaringhigh650]

you are still breaking the law.


Then I'd rather break the law than put my aircraft in danger. :}

[BackPacker]

If you need to do it, you still need written permission, or you are still breaking the law.

Well, it doesn't sound like a one-off problem. Wouldn't Mr. Boeing already have talked to the CAA and gotten written permission for the whole 747 fleet, pointing to the AFM, SOPs and such?

If so, the written permission is likely to be lurking in a bottom drawer somewhere.

[ShyTorque]

Then I'd rather break the law than put my aircraft in danger

In an emergency I would do too.

But from what has been written, that isn't the case here. It appears to be pre-planned and carried out on a not uncommon basis.

It's unprofessional to be ignorant of the laws of any country over which we fly, or choose to ignore the ones we don't like, especially if safety may be compromised.

By that I mean the safety of others. The 250 kt rule is one of those rules and needs to be complied with irrespective of how heavy the commercial burden to the company to disregard it.

[lucaberta]

I'm sorry but I fail to see the point in all of these messages about "breaking the law" on the 250kts below 10000' in the UK, when you, ShyTorque, said this a few messages ago:

SNS3Guppy, The reason I asked was because under UK aviation law, you need a written permission to go above 250 kts in Class G airspace. I don't think ATC can give ad hoc permission on the day; Class G doesn't "belong" to ATC.I trust that SNS3Guppy and his/her 747 spend most of their time during their initial climb in IFR within CAS? Am I wrong?

Ciao, Luca

[mm_flynn]

Well, it doesn't sound like a one-off problem. Wouldn't Mr. Boeing already have talked to the CAA and gotten written permission for the whole 747 fleet, pointing to the AFM, SOPs and such?

If so, the written permission is likely to be lurking in a bottom drawer somewhere.
As I said earlier, there can't be many (if any) UK airports that cater to fully loaded 747s that are not either in controlled airspace to the ground or have a very short link into controlled airspace. As such, this rule with regard to Guppy's comments would seem to be a non-issue in the UK. (military excepted, who clearly operate on occasion well above 250 knots down low)

Filton was mentioned, but is unlikely, with only 2.7kms of runway, to be handling a fully loaded 747 ;). Manston is the only civil airport I could think of with a long runway and no controlled airspace (and even this is about 1200 metres less than LHR in ASDA) and no direct connection to controlled airspace. So I guess if anyone ever planned to operate heavy 747s out of Manston they would need to get the permission.

[ShyTorque]

I trust that SNS3Guppy and his/her 747 spend most of their time during their initial climb in IFR within CAS? Am I wrong?

lucaberta, Yes, but my concern was this quote:

As a function of our IFR clearance, and our operations specifications issued to us by the government agency that oversees our operation...which includes operation in uncontrolled airspace. We don't ask what type of airspace we're in during a flight under IFR. We don't particularly care if it's class A, B, C, D, E, G, etc.

If you're asking about the permission for making a high speed climb, the requirement for 250 knots below 10,000, where applicable, applies only if the minimum safe speed of the aircraft is below 250 knots. We don't need permission. We "ask" as a courtesy, for a high speed climb, but also often simply inform ATC about our climb speed. ATC expects it. The one exception will be times when a departure procedure may require a slower speed and a configuration change may be delayed because until those requirements are met, the SID or DP establishes the speed. At that point, our primary concern is meeting climb gradients and crossing restrictions.

An IFR clearance should normally keep an aircraft inside CAS. If a pilot is unable to comply with the clearance or SID for any reason, ATC would need to be informed.

My concern is that from what was written earlier, in some cases this apparently may not happen due to the lack of performance of a heavily laden aircraft. Pilots were warned that aircraft flying in Class G, and not in contact with the same ATC unit should be aware that they may suddenly be confronted with a "heavy" leaving CAS at 300kts below 10,000ft. The UK ANO specifically forbids this. SNS3Guppy seemed to be unaware of this UK rule. If he doesn't operate in UK (but I think he possibly does, after all the thread was opened by a UK poster) then I don't give two hoots.

[soaringhigh650]

An IFR clearance should normally keep an aircraft inside CAS

But if you look at most airports across the UK, a lot of IFR flight is done outside CAS. Some say this is a problem. Some don't.

[lucaberta]

ShyTorque, thanks for the clarification.

It would be interesting to know precisely in which specific case the issue of seeing an initial climb of a heavy B747 would infact fall outside CAS in the UK, mm_flynn mentioned Manston but I don't believe they operate a cargo facility there. I've seen B747 doing training flights there, yes, but that's a whole different ball game.

Interesting to see the difference between the FAA and the CAA interpretation of the rule. The FAA includes the "safety" aspect in the speed limit (which is also valid inside of CAS) whereby the UK only have outside CAS, and I guess it's mostly for see-and-avoid timing for aircraft with high closure rates.

Ciao, Luca

[ShyTorque]

Re. See and avoid, look back to the post by Roffa who quoted from MATS. There are a number of choke points in UK where a heavy failing to keep within CAS could cause a danger, especially at high speed.

[SNS3Guppy]

My concern is that from what was written earlier, in some cases this apparently may not happen due to the lack of performance of a heavily laden aircraft. Pilots were warned that aircraft flying in Class G, and not in contact with the same ATC unit should be aware that they may suddenly be confronted with a "heavy" leaving CAS at 300kts below 10,000ft. The UK ANO specifically forbids this. SNS3Guppy seemed to be unaware of this UK rule. If he doesn't operate in UK (but I think he possibly does, after all the thread was opened by a UK poster) then I don't give two hoots.

I do operate in UK airspace, actually, and I am aware of the regulation. Perhaps you're not.

http://www.caa.co.uk/docs/33/CAP393.pdf
CAP 393, Section 4, Rule 21:

Speed limitations
21 (1) Subject to paragraph (2), an aircraft shall not fly below flight level 100 at a speed which, according to its air speed indicator, is more than 250 knots.
(2) Paragraph (1) shall not apply to:
(a) flights in Class A airspace;
(b) VFR flights or IFR flights in Class B airspace;
(c) IFR flights in Class C airspace;
(d) VFR flights in Class C airspace or VFR flights or IFR flights in Class D airspace when authorised by the appropriate air traffic control unit;
(e) an aircraft taking part in an exhibition of flying for which a permission is required by article 80(1) of the Order, if the flight is made in accordance with the terms of the permission granted to the organiser of the exhibition of flying and in accordance with the conditions of the display authorisation granted to the pilot under article 80(6)(a) of the Order;
(f) the flight of an aircraft flying in accordance with the A Conditions or the B
Conditions; or
(g) an aircraft flying in accordance with a written permission granted by the CAA authorising the aircraft to exceed the speed limit in paragraph (1).
(3) The CAA may grant a permission for the purpose of paragraph (2)(g) subject to such conditions as it thinks fit and either generally or in respect of any aircraft or class of aircraft.

You've suggested several times that the only method of being exempt from the 250 knot speed limit below 10,000 is to have a written authorization, and this is untrue. That is one possibility, but only one of 7 categories of exemptions from the 250 knot limit.

Is the average private pilot willing to bet his life on compliance or a private pilot's understanding of the regulation, such that he or she insists on "sticking his fin" into the overlying airspace?

Take the case presented earlier of an engine failure after takeoff. We're goin to level at a predetermined altitude, to which ATC is not privy, and we are going to execute a turn procedure, also to which ATC is not privy. We will be dealing with cleaning up the airplane, accelerating to our clean airspeed on that heading (as given in our turn procedure), and then dealing with the problem (or problems), without any particular knowledge regarding the local airspace. In short, we have zero concern whether the airspace is class this or that, controlled or uncontrolled, and we'll resume working with ATC once we have handled our situation and stabilized it. Period.

Rarely is a high airspeed required when descending below 10,000, but in our case, nearly always it's required when climbing away after takeoff. We fly that higher airspeed too, including in the UK, given operational necessity. The only exception will be found in situations when a departure procedure requires a lower speed, such as is often the case in South America (185 knots is common), for the sake of turn radius.

Wouldn't Mr. Boeing already have talked to the CAA and gotten written permission for the whole 747 fleet, pointing to the AFM, SOPs and such?

Mr. Boeing is long since dead, but no, it doesn't work that way.

No need for SOP's. We have our performance, it's well known, and any operator of the type, including British Airways, will fly the minimum speed in the climb when heavy, which is V2 + 100 knots, and always above 250 knots. As a condition of operation, we do not need to abide a 250 knot limit in the climb, and we don't. Further, the regulation doesn't require us to.

During an emergency, of course, we will meet the needs of the emergency as necessary, which will include the clean climb.

If you need to do it, you still need written permission, or you are still breaking the law.

No, we don't require written permission, and no, we're not "breaking the law."

Again, I refer back to your prior statement that "I know one corporate operator who was warned off and threatened with further action for flying over 250kts." This operator did not have an operational need to be climbing above 250 knots, nor a clean speed above that value, did he or she? Of course not.

You seem very hung up on the possibility of a large airplane penetrating airspace frequented by light airplanes. Let me remind you that this is but one example of why vigilance is appropriate, especially when operating near controlled airspace. Those who would insist on their right to "stick their fin" in controlled airspace by flying directly against it should be made aware of potential conflicts. Despite your preoccupation with my clean climb speed, it's but one of many possible sources of conflict. My speed isn't the issue here; it's one's approximate operation to areas of potential conflict.

Yes, mode C may be inaccurate, especially in airplanes typically flown only under VFR. Yes, ATC compensates for standard atmosphere QNE output product from mode C, by correcting for QNH. Yes, altimeters can be off, and yes, separation can suffer. Yes, you can encounter conflicts with high speed and large traffic close to, and either side of boundaries, and no, you should never count on those boundaries to offer any sort of protection. One should not assume that one is free of trouble while skirting the ragged edges of busy airspace (or controlled airspace, for that matter).

No matter what your action or intent, always give yourself a margin, a buffer. You may need it. Plan accordingly.

[Roffa]

SNS3Guppy,

Forgetting emergencies and unusual stuff, this interested me...

No need for SOP's. We have our performance, it's well known, and any operator of the type, including British Airways, will fly the minimum speed in the climb when heavy, which is V2 + 100 knots, and always above 250 knots. As a condition of operation, we do not need to abide a 250 knot limit in the climb, and we don't. Further, the regulation doesn't require us to.

If you were flying out of LHR, where the SID says "Maximum 250KIAS below FL100 unless otherwise authorised" do you feel you should say anything to ATC before accelerating above 250kts?

ATC certainly would expect you to as the departure separations are predicated on the speed limit and I don't believe a BAW flight would accelerate without saying anything.

[SNS3Guppy]

If you were flying out of LHR, where the SID says "Maximum 250KIAS below FL100 unless otherwise authorised" do you feel you should say anything to ATC before accelerating above 250kts?

You bet, and we always notify ATC, as previously described in this thread.

[ShyTorque]

SNS3Guppy,

I'm fully aware of the rules regarding speed limitation, which is exactly why I asked you the question; I could just as easily have copied and pasted your quote from CAP393. You haven't answered my original question so here it is again:

Under what written permission do you exceed the 250 kt rule outside CAS?

In your earlier post you said that you do not care in what type of airspace you fly above 250 kts. Whatever your personal interpretation, the ANO is legally binding. It may not be in your place of main residence, but in UK it most definitely is. If you don't comply, you break the law.

A SID keeps the aircraft inside CAS, there is no issue over that.

I don't think anyone else intends to, nor mentioned "sticking a fin" into CAS, which is obviously highly stupid because as well as being dangerous it will always be investigated in depth and the CAA do prosecute pilots for CAS infringements.

However, UK airspace is very constricted in places and aircraft do need to fly in Class G under control areas. They have been given the perfectly legal right to do so. In much of the area under the London TMA, MSA sits right under the CAS (the CAS was designed that way) so it must be expected that aircraft will fly under the lower limit.

Sometimes transitting aircraft will be on a different frequency to the traffic climbing from an airfield, in fact this is commonplace around the London area. Not ideal, but that is the way it works.

If your heavily laden aircraft cannot make a normal climb gradient without breaking the terms of the ANO because it is unable to follow a SID inder normal circumstances, it must be overloaded.

Btw, I'm not merely presenting my personal interpretation of the ANO. Only last Friday I was under a traffic service in Class G and overheard a pilot ask the Birmingham Radar controller:

"Are you happy with our present airspeed".

Reply: "What airspeed do you have?"

"300 kts" came the pilot's reply.

The controller replied: "Not above 250 kts below FL100". I don't know if the other pilot was climbing or descending, or if he was inside or outside of CAS but that is irrelevant.

[Jumbo Driver]

SNS3Guppy,
ATC certainly would expect you to as the departure separations are predicated on the speed limit and I don't believe a BAW flight would accelerate without saying anything.

Of course, this is quite correct. The 250kts below 10,000' speed restriction in UK is normally relaxed for an "operational requirement". This means that a 747 departure at high weight (for example) will need to climb at greater than 250kts when clean to maintain an optimum climb gradient, as part of the normal operational profile. ATC will normally approve this as a matter of routine but it is a professional requirement to advise ATC at the time of requesting/receiving clearance.

In the example of a departure from EGLL at high weight, the EGLL departure procedures cover this in exactly this way, viz.

6. Departure Procedures
a. Standard Instrument Departure (SID) procedures for aircraft departing from London Heathrow Airport are detailed at AD 2-EGLL-6-1 to 6-7 and incorporate the Noise Preferential Routes (NPRs) detailed in AD 2.21.
b. Speed Limit: In order to optimise the departure flow and assist in the separation between successive departing aircraft a speed limit of 250 kt is applicable unless removed by ATC. ATC will endeavour to remove the speed limit as soon as practicable after departure, using the phrase ‘No ATC Speed Restriction’. Pilots are reminded that this phrase does not relieve the pilot of the responsibility to adhere to the ground track of the Noise Preferential Route, which may require a speed/power limitation.
c. If for any reason pilots are unable to comply with the 250 kt IAS speed limit the pilot should immediately advise ATC and state the minimum speed acceptable. If a pilot anticipates before departure that he will be unable to comply with the speed limit he should inform ATC when requesting start-up clearance, stating the minimum speed acceptable. In this case the pilot will be informed before take-off of any higher speed limitation.
Can we now get back to something more interesting and closer to the thread topic, please ... ?

JD
:)

[SNS3Guppy]

I don't think anyone else intends to, nor mentioned "sticking a fin" into CAS, which is obviously highly stupid because as well as being dangerous it will always be investigated in depth and the CAA do prosecute pilots for CAS infringements.

Perhaps you missed post #64, when Mar1234 said "When the controlled airspace is 1000ft AGL, and I have little hope of a clearance, you can bet your last dollar I'm going to have my fin right up against the bottom of it. You might equally well ask why the professionals need fly so close to the base, and the wild, uncontrolled chaos that reigns beyond.."

The controller replied: "Not above 250 kts below FL100". I don't know if the other pilot was climbing or descending, or if he was inside or outside of CAS but that is irrelevant.

AGAIN, that controller wasn't speaking to an aircraft with a clean speed higher than 250 knots, was he?

He was not.

AGAIN, you're obsessed with the issue of speed here, when there are many aspects to the conversation, and the topic is mode C, not speed. You appear to have globbed onto one example, and appear to be unable to let it go.

Can we now get back to something more interesting and closer to the thread topic, please ... ?

Quite so.

[ShyTorque]

AGAIN, that controller wasn't speaking to an aircraft with a clean speed higher than 250 knots, was he?
He was not.


How do you know?

SNS3Guppy, you seem to be unable or unwilling to provide any answer to my question and prefer to divert the issue so regrettably, it appears pointless to discuss it further.

I'll have to assume you have no written permission.

[SNS3Guppy]

If I'm exceeding 250 outside controlled airspace, you can bet at the time, I don't need written permission. Within controlled airspace, of course, it's not an issue.

You really are obsessed with this, aren't you?

Would you prefer to make this thread about airspeed, or will you address mode C?

How do you know?

Simple. The controller wouldn't have blinked twice if the speed above 250 was for operational necessity. Like your previous irrelevant example, you're discussing an aircraft that had no need to operate above 250 knots, and clearly so.

Are you able to return to the topic, or will you belabor this obsession of yours any further?

[ShyTorque]

I've already said I saw no point in discussing it further.

Thank you; now let's leave it there.

[soaringhigh650]

I've actually enjoyed watching this bickering! :ok:

[Roffa]

Can we now get back to something more interesting and closer to the thread topic, please ... ?

I think you dropped your badge :)

http://gallery.mtbr.com/data/mtbr/500/thumbs/thread_police_badge.jpg

[Jumbo Driver]

Ho ! Ho !

JD
;)



That's quite good ... for ATC ...