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…

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 and AIP ENR 1.6.2 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]

Originally Posted by 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?

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

Safety-Critical Mailing List Archive 1999: TCAS near-miss: "an error in just one wire making it 1-0-1"

[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