How accurate is your Mode C?

[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]

Mode C, Infringements etc.

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]

How accurate ?

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!

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...

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

[A and C]

Lucaberta

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

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

[lucaberta]

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

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.