Mode A or C during aerobatics

[apanxx5]

Hi,

When ever I enter a runway I select mode C on the transponder.
During aerobatics should I select mode A or stay on C.

I have always used mode C but was told on the weekend I should select A.

Or it just does not matter?

Thanks

[DFC]

Use mode C.

Your transponder's encoder just like your altimeter will continue to provide useful information while you aerobat.

Will assist ATS and ACAS equipped aircraft.

Regards,

DFC

[HEATHROW DIRECTOR]

But remember that to ATC the altitude will be unverified unless you check the height readout with an ATC unit at the time.

[vector4fun]

I have always used mode C but was told on the weekend I should select A.


I'm curious who told you that and why. Your profile says CA; in the U.S., a lot of verticle manuvering will cause our ATC computer to disregard your Mode-C and show XXX instead of your actual altitude. Still, any time you're not exceeding around 3000 fpm, we should still be able to read it. I'd prefer you have it on in my airspace.

[apanxx5]

Vector4fun,

Hi,

I only aerobat "on the weekends" so that was what I was trying to say.

I was told to have mode A on when aerobating by another pilot (with no ATC experience). He said that it could confuse ATC:confused:

Thanks for the advice guys

[nibog]

It depends.

On the newer radar systems with ARTAS, the computers can calculate ROC/ROD as part of ths short term conflict alert(STCA), and may provide spurious "level bust" warnings to the controller.

This happens because the radar systems are configured for IFR commercial traffic and as such operate within pre-defined limits. Traffic climbing through 7000' at 4000'fpm may trigger an alert with cruising traffic at 10,000' because the computer predicts that the climbing traffic may bust the level cleared to..

If you're doing aerobatics in controlled airspace or over an aerodrome, I'd prefer you have mode C off. This is because another aircraft will not come near you as the controller shouldn't allow it.

If you're performing in un-controlled airspace, leave mode C on. It will aid the controller in providing traffic information to the pilot who after being warned of the activity still wants to fly through (there's always one). Also useful as an aid to collision avoidance for TCAS equipped VFR aircraft in uncontrolled airspace.

[d192049d]

In addition should we Squawk 7400??

M

[av8boy]

Do you mean 7004?

Anyway, not in the US. 1200 with Mode C, please. HD is, of course, correct, the Mode C is unverified. However, I'd MUCH rather see an unverified Mode C than nothing at all.

Dave


Welcome aboard apanxx5! I was in your neighborhood just last week!

[chevvron]

If you have mode C fitted, then always use it.
Use 7004 when aerobatting unless you're in contact with an ATC unit who allocate another code.
7004 converts to 'Display' on radars at some RAF airfields (rather than 'Aeros' or something) and it scares the hell out of them!

[HEATHROW DIRECTOR]

<<However, I'd MUCH rather see an unverified Mode C than nothing at all. >>

Simply don't follow that at all... If it's unverified it is useless information. If a tiddler with a dodgy transponder starts doing aeros under CAS you surely can imagine the problems - ATC possibly taking avoiding action, TCAS going off all over.. . No way, Jose!

[zkdli]

If you have a TXPDR that is equiped for mode"c" you should use it. It is essential for TCAS to operate, it does not matter whether it is unverified or not. If you are operating under CAS and you enter it at the top of a loop for example and you dont show "C" and get in close proximity with a TCAS equiped jet, the first thing the pilot may know about you is seeing your face in his.

[vector4fun]

Also, in the U.S., FARs say Mode C shall be on at all times; unless they had a waiver for operations in their aerobatic box.

[av8boy]

If it's unverified it is useless information.

I disagree. An unverified Mode C is not helpful for official separation of aircraft—agreed. However, if I've got an aircraft I'm talking to at a particular altitude, and there's an unverified Mode C opposite direction (or crossing, etc) at the same altitude, I'm going to pass that traffic ("altitude indicates"). I've got 20 pounds that says that the vast majority of the aircraft drivers out there would like me to help them remain clear of that traffic if they don't see it.

PLEASE don't misunderstand, HD. I've got a great deal of respect for you (I've been at this for 23 years and, except for the fact that you’re in the UK, YOU might have trained the guy who trained the guy who trained me). I just see this as the difference between dogmatic and practical. Certainly we've all seen Mode Cs that were inaccurate. However, when I'm working somebody who is at the same altitude as an unverified Mode C, I'm pulling out all the stops. Especially since the VAST majority of Mode C readouts are pretty darned close. Now, I’m not saying that I depend upon an unverified Mode C to lull me into not calling traffic. What I’m talking about is the opposite… when the unverified Mode C is close to somebody I’m working, I’m all over it.

A brief anecdote… Once upon a time I had a friend who worked at the same facility as I who had something bad happen. This was the approach control at a very busy international airport. On the day in question there was lots of VFR traffic without Mode C showing up on his display. He was working arrivals in (what was then referred to as) the TCA. The non-Mode C traffic below the TCA (which was not talking to him, nor was it required to talk to him) was very, very heavy. He said they looked like ants swarming on an anthill. He assumed that the VFR aircraft were below the controlled airspace he was working and although he did call some VFR traffic for the IFR guys, he didn’t get them all. As he said, “when the elevator door opens, you expect there to be an elevator.” In other words, when there’s a solid mass of VFR aircraft being displayed, at some point you have to depend on them to stay where they are supposed to be. Such was not the case. One of those light VFR aircraft had blundered into the TCA and was struck by an arriving commercial aircraft, resulting in a great loss of life. If that VFR aircraft would have been Mode C-equipped, my friend would have pointed it out as traffic and this disaster might have been averted. Again, it’s not about seeing an unverified Mode C that appears to be 5,000 feet below your IFR traffic and ignoring it. It’s about the last, best chance to avoid metal-to-metal contact.

I am allowed to use a Mode C if it is accurate to within 299 feet. If the VFR in this anecdote had been showing an unverified altitude anywhere NEAR the IFR traffic (certainly anywhere near controlled airspace), this whole thing might have been a non-event. Hell, if he’d been showing any Mode C at all the traffic probably would have been called. In any case, my friend did what he was supposed to do, but it obviously wasn’t enough. That’s what he believes anyway.

The NTSB determined that the probably cause of the accident was the limitation of the air traffic control system to provide collision protection, through both air traffic control procedures and automated redundancy. Factors contributing to the accident were (1) the inadvertent and unauthorized entry of the [VFR aircraft] into the [TCA] and (2) the limitations of the “see and avoid” concept to ensure traffic separation under the conditions of the conflict. I’m just not clear on how an unverified Mode C would have been “useless” in this accident.

I’d only add that this facility was not the busiest in the US when it came to overall movements. However, it WAS the busiest when one divided the number of movements into the number of control positions ( this is to say, other TRACONs worked more aircraft, however, the number of aircraft per position was far greater at this facility). Even at this facility I was never too busy to call traffic and say, “altitude indicates…” Small price to pay given the alternative.

Dave

[chevvron]

av8boy:
I think most of us in the UK think/operate like you; it's just that some people have little or no RECENT experience of operating in airspace of class D or lower.

[DFC]

If you're doing aerobatics in controlled airspace or over an aerodrome, I'd prefer you have mode C off. This is because another aircraft will not come near you as the controller shouldn't allow it.

Lets stop fitting TCAS then cause the controller will not let two aircraft come close to each other. :)

Turning off safety systems (mode C, TCAS, Conflict Alert) can have disasterous consequences as we have seen.

Regards,

DFC

[nibog]

DFC. You may be mis-interpreting my original point.

I would never condone the switching off of useful safety nets. They are there for a reason. TCAS, like STCA, is a valuable aid and should be used if it's available, but we must remember it's lititations.

In some instances, however, it may not be practical to use all available aids. If I am working a high performance aircraft doing aerobatics between ground - 5000', then that will be the only local aircraft in the area. Other aircraft may be held at visual reporting points or routed around. I will have informed the relevent adjoining sectors and any airspace required will have been co-ordinated.

Under the circumstances described above, I will not require or desire mode C info from the performing aircraft. TCAS, great as it is, does not universally respect airspace boundaries. TCAS is designed to provide emergency traffic info & separation of IFR aircraft in situations where the system has failed and radar separation is lost. Pilots are briefed to obey TCAS RAs over controller instructions, so if an RA is triggered, it will most likely be obeyed. This could potentially have disastrous results if it is triggered as a result of an aerobatic climb. The aerobatic aircraft will most likely be okay, once they reach the top of climb they usually go back down again. But the IFR cruising aircraft that responds to the RA will go where...
... and hit what ... STCA prediction alerts that are triggered as a result of (rate of) aerobatic climb (loops etc) are a distraction, and can also be potentially dangerous.

I condone the turning off of mode C for aerobatics, ONLY in controlled airspace where alternative and sufficient precautions have been taken. The reason for this is so as NOT to trigger spurious STCA alerts or TCAS RAs.

If operating outside of controlled airspace, continue to use mode C.

[av8boy]

little or no RECENT experience of operating in airspace of class D or lower.

chevvron… As I indicated, it was not my intent to disparage HD. The event to which I made reference occurred almost 20 years ago. With all due respect, recent experience has nothing to do with it.

I mean no disrespect to you chevvron. We all have different ways of interpreting things. I would just say that I believe very strongly that HD has much to teach all of us. This was just a respectful disagreement between he and I. If it didn’t come across that way, I owe him an apology.

[Scott Voigt]

HD;

We also look at things a bit different in the US. First of all we don't differentiate with the RIS RAS and other such things <G>... You are either IFR or VFR getting some service. As to the mode C, we use it period. For IFR it must be verified for IFR use, however the FAA has done away with the not verified phraseology completely due to them feeling that mode C is mostly accurate. One of the things that we as controllers are required to do is if we see a mode C that is in error, we are requred to report it so that Flight Standards may have a chat with the aircraft owner about getting it fixed... They are fairly serious about that and it came about after the advent of TCAS. Something to do with some of the VFR's flying about with mode C showing FL315 and scaring the living blazes out of a flight crew...

Take care

Scott

[DFC]

Nibog,

You are obvously talking about Class B airspace (where VFR flights are separated from VFR flights).

In anything less, VFRs are not separated from VFRs and in D or E controlled airspace, VFRs are not separated from IFRs. Thus assuming that the aerobatic flight is VFR :) I take it that you would only ask a pilot to turn off verified mode C during aerobatics in class B airspace.

Do you work in Belgium?

--------

Scott,

The problem in the UK is that incorrect mode C readouts are not reported and if they are the reports are not followed up unless an incident or accident occurs.

------------

Very ironic that I have to be told that the permit to fly aircraft converging on me at the same level has an unverified mode C...............Heck, the altimeter the pilot is looking at was probably picked up on a stall at some car boot sale and is not required by the airworthiness requirements to be calibrated! Seems to me that the mode C may be the only true indication of level!

Regards,

DFC

[nibog]

Class C Airspace. Sorry to dissapoint.

I don't usually separate VFR from VFR. But, with class C being controlled airspace, and me being the controller, I do provide extra protection for Aerobatics and Parachute Ops etc..

Regardless of the Airspace Category and VFR requirments, I don't think it very wise to fly through (or allow an aircraft to fly through) an area of (Controlled) Airspace that has aerobatics in progress.

Also, an unverified Mode C is not necessariy wrong, it is just as it sounds - Not verified as being correct on this flight at this time. If a mode C is discovered as being outside of it's limits in flight, the pilot is requested to turn off mode C. It's nothing to do with equipment calibration.

Altimiters are calibrated on every flight prior to departure - what does the pilot do with the QNH?

[DFC]

Nibog,

Have you experience of aircraft doing aeros with mode C on causing problems? i.e. Military flights or similar that may be operating in airspace below which your flights are operating?

We reguluarly pass close to large military danger areas which are within controlled airspace. If flights within those segrated areas were told to switch off mode C we would have serious safety concerns because it has happened that flights have exited the airspace both vertically and laterally and without mode C, the ACAS system and your STCA has no way of drawing our attention to a possible collision.

I don't think it very wise to fly through (or allow an aircraft to fly through) an area of (Controlled) Airspace that has aerobatics in progress

I agree that it is unwise for a pilot to fly through and area where a pilot is performing aerobatics. However, in general that is a pilot decision and the chosen separation by a VFR flight against another VFR flight is up to the pilot.

It's nothing to do with equipment calibration.

Altimiters are calibrated on every flight prior to departure - what does the pilot do with the QNH?

As you are no doubt aware the encoder that provides the level information to the transponder is permanently set to 1013.2. Setting QNH in the aircraft makes no difference to the level information transmitted by the transponder. Your supervisor or engineers have to imput your QNH into your system so that the computers can work out and display altitudes when appropriate.

Setting QNH on the altimeter pre-flight and checking the indication against the elevation of the apron / threshold / airfield ensures that the aircraft's pressure altimeter is correct at that level. It does not in any way check the encoder which supplies information to the transponder.

Just because the altimeter indicates the correct airfield elevation with QNH set does not guarantee that the altimeter will indicate 5000ft when the aircraft reaches 5000ft even if it is ISA.

Regards,

DFC

[nibog]

Simple answer is Yes. Mode C does sometimes cause predicted STCA alerts while Vertical Separation is still maintained. Maybe our Radar is a bit over-cautious - maybe not - that's not really for me to answer. The alerts would not be an everyday or a regular occurence and are not related to aerobatic activity. Our Aerobats don't get close enough to IFR traffic to cause problems. Our IFR / IFR do alert sometimes due to high ROC/ROD while still separated. I've yet to come across a false TCAS RA, and it is possibile that the extra precautions may have contributed to us not having had one.

Without getting too tit for tat .....

1. When the pilot adjusts the altimiter settings and it's checked and indicates airport elevation, that's the "car boot sale" altimiter calibrated. Of course this doesn't check the encoder - thats what we do with verification.

2. If, as you suggest, a pressure altimiter is correct at aerodrome elevation and incorrect at higher altitudes, what will happen? If there is no backup system he'll climb till reaching his desired indicated altitude. ATC will check his alt if it's different from the cleared level and if an error is found, it's the mode C that'll be turned off.

3. Yes, you're correct, Mode C encoders use QNE/1013.2 at all times and the Radar data processors will decode the data & display Alts or FLs as required. Next time in radar, watch a mode C as it passes the Trans Layer.

4. The mode C data is checked when it is verified by the controller, which again, does not mean that an unverified mode C is wrong - it just means that it hasn't been checked yet. If it has been checked and is found to be wrong, it can be turned off and no longer cause confusion. 9 out of 10 unverified mode Cs in uncontrolled airspace are within limits, but just not checked yet. Use it unverified only as an aid to VFR traffic info but not as a method of IFR separation. eg, Unidentified Tfc, 11 O'clock, 2miles, indicates 1200' unverified.

5. Now I think we all agree that mode C should be used if it's available. The majority of our aerobatic activity uses mode C, but some does not. Generally, it's the Gnd-5000' over the airfield that does not, and the Remote Area FL100- FL300 that does. When it's not used, it's because of genuine reasons and extra precautions will have been taken.


And entertaining as it is :D , I don't see any need to argue these points any further. I think ATC everywhere uses the same general rules with regard to mode C verification and use.