ATC and Mode C inaccuracy

[azure]

Okay, this evening I was up shooting approaches with my CFII. We were IFR because of scattered low clouds and forecast of very MVFR ceilings. After we were done, returning to home base at 3000, the approach controller came on and said he needed us "AT 3000", which I acknowledged. The issue was separation from traffic in the Bravo above us (we were under a 4000 foot shelf), CFII then queried him as to what our Mode C was showing, and he said 3200, at one point 3300. The highest I had been at any point was 3060. At that point we both realised that we were squawking about 200 feet high and my CFII ordered me to get down quickly to 2800, in fact he took the controls without saying anything. I meekly suggested that I thought the controller was supposed to query our altitude, not assume the Mode C is correct, and that our altimeter was fine. But he said he was more concerned about avoiding a bust. Later when we were cleared for the approach to home base and told to "maintain 3000 until established", I dutifully held us at 2800 instead. Okay, the MSA is 2700 so no obstacle clearance issues, but still... that did not sit well with me.

Clearly our Mode C encoder is out of, or nearly out of tolerance and needs checking. But what's the best way to handle this kind of situation in the air? My understanding is that ATC is supposed to say "say altitude", which my CFII effectively answered without being asked by saying that our altimeter read 3000. So isn't the worst that ATC could have done to tell us to "stop squawk altitude" and/or get our Mode C fixed? I can't help thinking that by taking us down to where we were squawking 3000 we were making it look even more as if an altitude deviation had occurred. Or could they still bust him for flying with out of tolerance Mode C?

BTW our pressure altimeter had been recently overhauled and our home base has an AWOS so we knew for certain that our altimeter was within tolerance, and we were keeping the altimeter setting updated as reported by ATC.

 

[BillTIZ]

Did the controller issue an altimeter pressure setting? "Local Area Altimeter 29.85."
If you set what he said, and flew what your Altimeter said was 3000 and Told him and then the Mode C reported to him and displayed 3200. Then he has to accept your 3000 Report and should ask you to "stop squawk Mode C".

I don't think diving to 2800 to "avoid a bust" is the best thing. You are showing 3000, the controller has to accept your report at 3000.

Earlier in the flight at the first "radar contact", the controller should have verified your altitude reported with the Mode C display. If he doesn't, he cannot use ModeC to seperate traffic. If he did and it "appeared to be correct" then, then why is it 200ft off now?

I've known ATC radars to momentarily show bad Mode C altitudes because a bad pressure setting was loaded into the computers, but it would show the same error for all aircraft. Not just yours.

I would have the local shop check the transponder/ModeC reporting.

If ATC assigns 3000 and you are within +/- 200ft of assigned altitude then all he will see is your code/callsign (depending on Center or Approach Radar) and your altitude with a C "30C" if it reports above, he would get 30(down-arrow)32. Also your target data tag and the traffic at 4000 would flash if you were within or were projected to get within 3nm (Approach Radar) laterally.

 

[sba55]

I agree with Bill. Maintaining a lower altitude wasn't a good call at all, especially if you're in IMC and vertically separated from other traffic.

Instead, I'd verify the altimeter setting and, if that doesn't fix the discrepancy, tell ATC what's going on. Deviating from your assigned altitude can be quite dangerous in IMC.

-Felix

 

[azure]

Did the controller issue an altimeter pressure setting? "Local Area Altimeter 29.85."

Yes, that's what I meant when I said that we were keeping our altimeter setting "updated as reported by ATC". Sloppy wording on my part, sorry!

If you set what he said, and flew what your Altimeter said was 3000 and Told him and then the Mode C reported to him and displayed 3200. Then he has to accept your 3000 Report and should ask you to "stop squawk Mode C".

That's what I thought, too. The controller repeated the altimeter setting, I verified that we had it set correctly and I assume my instructor did too, and he reported our altitude reading. My CFII was very apologetic, the controller didn't pursue the issue further, and I felt CFII push the nose down, and down we went to 2800.

I don't think diving to 2800 to "avoid a bust" is the best thing. You are showing 3000, the controller has to accept your report at 3000.

Thanks, that confirms what I thought...

Earlier in the flight at the first "radar contact", the controller should have verified your altitude reported with the Mode C display. If he doesn't, he cannot use ModeC to seperate traffic. If he did and it "appeared to be correct" then, then why is it 200ft off now?

No, he never verified our altitude. We were holding all assigned altitudes as well as I could (which got better with time as my "nerves" over being for-real IFR for the first time went away). I'm sure our altitude never appeared to be correct, but the controllers never said a thing until it looked as if there might be a loss of separation, based on our Mode C. And at that point, this one assumed we were busting altitude and was actually quite irate.

I would have the local shop check the transponder/ModeC reporting.

Yup, if it was my decision, it would go in tomorrow. Knowing the way things get done in this club, I may have to switch planes to finish my training.

 

[azure]

I agree with Bill. Maintaining a lower altitude wasn't a good call at all, especially if you're in IMC and vertically separated from other traffic.

Instead, I'd verify the altimeter setting and, if that doesn't fix the discrepancy, tell ATC what's going on. Deviating from your assigned altitude can be quite dangerous in IMC.

-Felix

Yes, that's what I thought too. In this case conditions actually turned out to be VFR and there was no IFR traffic below us. I doubt my CFII would have done that in IMC. But even in VMC it seemed a really strange over-reaction and I didn't see the point of it, considering that he had reported our altimeter reading and had no reason to consider it unreliable.

This isn't the first time ATC has not queried our altitude BTW, just the first time under IFR. A few nights ago we were shooting approaches at a different airport and I was at 2800 feet and the controller said to maintain 3000, clearly thinking that we were already at 3000. We've been in the grip of persistent high pressure and I thought, it must be that difference between actual altitude and pressure altitude. Driving back tonight I thought about it and realised that the discrepancy would be in the other direction -- if the altimeter setting is greater than 29.92, pressure altitude is lower than true altitude. So our Mode C is definitely reading high.

Maybe my CFII just forgot that our altimeter was fine and panicked, thinking we had busted altitude. I don't want to "Monday morning quarterback" him, I just wanted to know whether the way he handled the situation was correct. I really felt it was wrong for a couple of reasons and didn't want to follow this example without knowing for sure it was correct. Now I'll know not to.

 

[Let'sgoflying!]

The worst of it is, when you told atc that you were at 3K, (while he is seeing 3.3 on your tag) then a few minutes later he is seeing 3.0, he is going to nod his head, 'uh-huh, they were too high'.

 

[roncachamp]

Okay, this evening I was up shooting approaches with my CFII.

You should not do that, approaches are government property.

We were IFR because of scattered low clouds and forecast of very MVFR ceilings. After we were done, returning to home base at 3000, the approach controller came on and said he needed us "AT 3000", which I acknowledged. The issue was separation from traffic in the Bravo above us (we were under a 4000 foot shelf), CFII then queried him as to what our Mode C was showing, and he said 3200, at one point 3300.

Then it's a bit odd that the controller mentioned it, a Mode C altitude readout is considered valid when it varies less than 300 feet from the reported altitude. A single hit 300 feet off can reasonably be ignored.

The highest I had been at any point was 3060. At that point we both realised that we were squawking about 200 feet high and my CFII ordered me to get down quickly to 2800, in fact he took the controls without saying anything.

That was a bad idea, now you're deliberately off your assigned altitude.​

I meekly suggested that I thought the controller was supposed to query our altitude, not assume the Mode C is correct, and that our altimeter was fine. But he said he was more concerned about avoiding a bust. Later when we were cleared for the approach to home base and told to "maintain 3000 until established", I dutifully held us at 2800 instead. Okay, the MSA is 2700 so no obstacle clearance issues, but still... that did not sit well with me.

Kudos to you and ten demerits for your CFII. When the controller said he was showing 3200', I'd have responded with the actual altitude and the altimeter setting that was being used. If the setting was correct and the reported altitude differed less than 300' from the Mode C readout you were good to go. If it differed by 300' or more the controller should have told you to stop altitude squawk.​

 

[roncachamp]

Did the controller issue an altimeter pressure setting? "Local Area Altimeter 29.85."
If you set what he said, and flew what your Altimeter said was 3000 and Told him and then the Mode C reported to him and displayed 3200. Then he has to accept your 3000 Report and should ask you to "stop squawk Mode C".

That's not correct. If the reported altitude and Mode C readout differ by 200 feet or less the Mode C is good. If they differ by 300 feet or more the controller should state, "stop altitude squawk, altitude differs by XXX feet".

 

[Geico266]

Will there come a day when we use altitudes based on GPS reportings? The GPS is much more accurate and do it does not care about pressure.

Any thoughts from the POA crew?

 

[poadeleted20]

From a legal perspective, there's no "bust" if you're at the assigned altitude but your Mode C says otherwise as long as you verbally report accurately the altitude you're at. OTOH, what your instructor did (descend to make the Mode C read the assigned altitude) is a violation and compromises safety. And any CFI-IA should know both these things. You might want to remember that if your instructor gives you further bad ideas down the line.

 

[azure]

The worst of it is, when you told atc that you were at 3K, (while he is seeing 3.3 on your tag) then a few minutes later he is seeing 3.0, he is going to nod his head, 'uh-huh, they were too high'.

I thought that's what I said in my first post? Not in those exact words, but that's what I was thinking, it made us look guilty. Exactly the opposite of what my CFII wanted.

 

[roncachamp]

Will there come a day when we use altitudes based on GPS reportings? The GPS is much more accurate and do it does not care about pressure.

GPS altitude is more accurate than barometric altitude?

 

[poadeleted20]

GPS altitude is more accurate than barometric altitude?

Generally, yes, at least with WAAS -- it usually provides a better indication of true altitude above MSL than a barometric altimeter, which does not correct for vertical pressure variations, wind effects, or nonstandard temparature. Usually, the difference increases with altitude, so the difference is usually neglible down low unless it's incredibly hot or cold, or you have a howling wind over a ridge or something (see http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001207X04839&key=1 for more on that issue). However, ATC separation is based on barometric altitude, so you should not "correct" your altitude based on GPS indications, which could compromise that separation which based on everyone being at the same "wrong" altitude computed using the same altimeter setting.

 

[azure]

You should not do that, approaches are government property.

Groan.

Then it's a bit odd that the controller mentioned it, a Mode C altitude readout is considered valid when it varies less than 300 feet from the reported altitude. A single hit 300 feet off can reasonably be ignored.

Thanks Steven, so that means a difference of 200 feet or less is acceptable? It's quite possible that he saw us off by 300 feet more than once but didn't say so, since I was having trouble trimming the aircraft and did bob between 3000 and 3100 a few times, but I don't think I was ever above 3060. I'm embarrassed to say but trimming is something I've always found difficult. I'm working on it.

 

[roncachamp]

Thanks Steven, so that means a difference of 200 feet or less is acceptable?

Affirmative.


Order JO 7110.65S Air Traffic Control

Chapter 5. Radar

Section 2. Beacon Systems

5-2-17. VALIDATION OF MODE C READOUT
​

Ensure that Mode C altitude readouts are valid after accepting an interfacility handoff, initial track start, track start from coast/suspend tabular list, missing, or unreasonable Mode C readouts. For TPX-42 and equivalent systems ensure that altitude readout is valid immediately after identification. (TCDD-/BANS-equipped tower cabs are not required to validate Mode C readouts after receiving interfacility handoffs from TRACONs according to the procedures in para 5-4-3, Methods, subpara a4.)

a. Consider an altitude readout valid when:

1. It varies less than 300 feet from the pilot reported altitude, or

PHRASEOLOGY-
(If aircraft is known to be operating below the lowest useable flight level),

SAY ALTITUDE.

or

(If aircraft is known to be operating at or above the lowest useable flight level),

SAY FLIGHT LEVEL.

2. You receive a continuous readout from an aircraft on the airport and the readout varies by less than 300 feet from the field elevation, or

NOTE-
A continuous readout exists only when the altitude filter limits are set to include the field elevation.

[SIZE=-2]REFERENCE-
FAAO JO 7110.65, Para 5-2-23, Altitude Filters.
FAAO JO 7110.65, Para 5-14-5, Selected Altitude Limits.
FAAO JO 7210.3, Para 11-2-3, Display Data.[/SIZE]
​

3. You have correlated the altitude information in your data block with the validated information in a data block generated in another facility (by verbally coordinating with the other controller) and your readout is exactly the same as the readout in the other data block.

b. When unable to validate the readout, do not use the Mode C altitude information for separation.

c. Whenever you observe an invalid Mode C readout below FL 180:

1. Issue the correct altimeter setting and confirm the pilot has accurately reported the altitude.

PHRASEOLOGY-
(Location) ALTIMETER (appropriate altimeter), VERIFY ALTITUDE.

2. If the altitude readout continues to be invalid:

(a) Instruct the pilot to turn off the altitude- reporting part of his/her transponder and include the reason; and

(b) Notify the operations supervisor-in-charge of the aircraft call sign.

PHRASEOLOGY-
STOP ALTITUDE SQUAWK. ALTITUDE DIFFERS BY (number of feet) FEET.

d. Whenever you observe an invalid Mode C readout at or above FL 180, unless the aircraft is descending below Class A airspace:

1. Confirm that the pilot is using 29.92 inches of mercury as the altimeter setting and has accurately reported the altitude.

PHRASEOLOGY-
CONFIRM USING TWO NINER NINER TWO AS YOUR ALTIMETER SETTING.

(If aircraft is known to be operating at or above the lowest useable flight level),VERIFY FLIGHT LEVEL.

2. If the Mode C readout continues to be invalid:

(a) Instruct the pilot to turn off the altitude- reporting part of his/her transponder and include the reason; and

(b) Notify the operational supervisor-in-charge of the aircraft call sign.

PHRASEOLOGY-
STOP ALTITUDE SQUAWK. ALTITUDE DIFFERS BY (number of feet) FEET.

e. Whenever possible, inhibit altitude readouts on all consoles when a malfunction of the ground equipment causes repeated invalid readouts.

 

[azure]

From a legal perspective, there's no "bust" if you're at the assigned altitude but your Mode C says otherwise as long as you verbally report accurately the altitude you're at. OTOH, what your instructor did (descend to make the Mode C read the assigned altitude) is a violation and compromises safety. And any CFI-IA should know both these things. You might want to remember that if your instructor gives you further bad ideas down the line.

But I'm not sure whether he deliberately did that, or whether he wasn't sure whether our altimeter or our Mode C was to blame. I was sure it wasn't our altimeter because I've been checking it every time I copy an altimeter setting on the ground, ever since it was overhauled at annual a month ago (it had been reading about 60 feet high). And he may have realised that the controller wasn't following usual procedure by not asking us to verify altitude and just going by our Mode C. I really think he may have just panicked.

 

[poadeleted20]

But I'm not sure whether he deliberately did that, or whether he wasn't sure whether our altimeter or our Mode C was to blame.

Doesn't matter -- you don't do it that way, period, unless you're sure it's the altimeter and not the encoder.

I was sure it wasn't our altimeter because I've been checking it every time I copy an altimeter setting on the ground, ever since it was overhauled at annual a month ago (it had been reading about 60 feet high).

As any CFI-IA should know, altimeter errors like that are very consistent. You just don't see them suddenly jump from 60 feet to 200-300 feet off.

And he may have realised that the controller wasn't following usual procedure by not asking us to verify altitude and just going by our Mode C. I really think he may have just panicked.

Can't disagree with that, but I think panic is an inappropriate reaction for a CFI-IA to an altimeter error of that magnitude.

 

[Bob Noel]

Will there come a day when we use altitudes based on GPS reportings? The GPS is much more accurate and do it does not care about pressure.

Any thoughts from the POA crew?

The beauty of baro altitude is that the system is really quite
simple, simple to check, and not prone to jamming or electrical
failure. Baro altitude is much less expensive than GPS, never
mind WAA$.

 

[bobmrg]

Will there come a day when we use altitudes based on GPS reportings? The GPS is much more accurate and do it does not care about pressure.

Any thoughts from the POA crew?

The AIM disagrees with that, telling pilots specifically that barometric altitude should be used because GPS vertical error can be quite large (1-1-19(a)(4). Maybe this warning will disappear some day in recognition of WAAS, but right now it is The Word.

Bob Gardner

 

[azure]

We had another run-in this afternoon with a (the?) controller complaining about our Mode C indicating high. We were shooting approaches at a nearby field when the vis dropped and we had to get a pop-up to finish up and return to home base. On the way back we were at an assigned altitude of 3000 feet. No sooner had we reached that altitude than the controller started started scolding us because our Mode C was showing us at 3200. He went on and on about how there were jets above us in the Bravo (the base of the shelf is at 4000 MSL) and we were causing problems. I tried to stay calm and explained that our Mode C was known to read about 150 feet high and that we were indicating 3020 with an altimeter setting of 29.80, which was the current altimeter setting given to us by Approach. Dead silence from the controller. My instructor apologised for the faulty Mode C and asked if we should stop altitude squawk.

"No, don't do that, that would really screw things up" was the reply.

My instructor got nervous like before and started to descend us, asking the controller if it would be better if we went down to 2850.

Dead silence.

My instructor speculated later that the problem might actually have been due to the airliners' TCAS showing us as high. But I can't imagine anyone getting so worked up about that given that <=200 feet off is within Mode C tolerance.

We're having the transponder looked at and hopefully recalibrated tomorrow. I guess I'm just looking for advice on how to handle this situation in the future.

 

[BillTIZ]

My instructor got nervous like before and started to descend us, asking the controller if it would be better if we went down to 2850.

Dead silence.

Descending.. a really bad move.. most likely why the silence from ATC.
I thought we covered that once before?

At least ATC quieted down after you gave him the facts.
ATC should have told you to not descend.. most likely there was silence because he was referencing his minimum radar sector charts.

Stop ModeC would have been the easiest.. I have no idea why ATC thought it would be worse. Unless he forgot how to put in a "reported altitude" to the system.

 

[sba55]

My instructor got nervous like before and started to descend us, asking the controller if it would be better if we went down to 2850.

Did you mentioned to him last time that descending was not the proper or wise move in this situation? If so, I can't believe he did it again....

 

[azure]

Descending.. a really bad move.. most likely why the silence from ATC.
I thought we covered that once before?

Well we were IFR and my CFII was PIC. I did object, but my opinion doesn't count for much. As for this having been covered, he doesn't read this board.

In any case he stopped pushing and we leveled off when ATC didn't respond. I don't think we went down more than about 70 feet.

At least ATC quieted down after you gave him the facts.
ATC should have told you to not descend.. most likely there was silence because he was referencing his minimum radar sector charts.

Well, he didn't say anything, and whatever his problem was the facts from our end weren't enough to satisfy him. He was still pretty durned upset when he replied to my CFII... which was before we started to descend BTW.

Stop ModeC would have been the easiest.. I have no ide why ATC thought it would be worse. Unless he forgot how to put in a "reported altitude" to the system.

We were inside the Mode C veil, wouldn't it be a last resort to allow a target without an indicated altitude?

edit: Folks, I mentioned the fact that he briefly started to descend us just for completeness. Yes, I told him last time that I thought it was a bad move. Now could we please not get hung up on this? The reason I posted this is that I'm trying to understand what the controller might be thinking and why he might not have followed what I thought was accepted procedure, i.e. to instruct us to "say altitude" and either tell us to stop altitude squawk or make allowances for the discrepancy.

 

[roncachamp]

Folks, I mentioned the fact that he briefly started to descend us just for completeness. Yes, I told him last time that I thought it was a bad move. Now could we please not get hung up on this? The reason I posted this is that I'm trying to understand what the controller might be thinking and why he might not have followed what I thought was accepted procedure, i.e. to instruct us to "say altitude" and either tell us to stop altitude squawk or make allowances for the discrepancy.

What discrepancy? A Mode C indication of 3200' at the assigned altitude of 3000' MSL while on the correct altimeter setting is within limits.

 

[Greg Bockelman]

Now could we please not get hung up on this?

Well, the "hang-up" is that the CFII seems to have a gap in his understanding of how this is supposed to work. Not about anything you did or didn't do.

 

[roncachamp]

Well, the "hang-up" is that the CFII seems to have a gap in his understanding of how this is supposed to work.

As does the controller.

 

[N801BH]

I am assuming this "episode" is using the same plane?
If so then why didn't the owner get the encoder checked?
If it was checked and passed the test, tell ATC the plane is current and within limits and don't let him rattle your cage.
If it wasn't checked and you flew the same plane and got the same results your decision making skills need a reboot. IMHO YMMV.

Okay, this evening I was up shooting approaches with my CFII. We were IFR because of scattered low clouds and forecast of very MVFR ceilings. After we were done, returning to home base at 3000, the approach controller came on and said he needed us "AT 3000", which I acknowledged. The issue was separation from traffic in the Bravo above us (we were under a 4000 foot shelf), CFII then queried him as to what our Mode C was showing, and he said 3200, at one point 3300. The highest I had been at any point was 3060. At that point we both realised that we were squawking about 200 feet high and my CFII ordered me to get down quickly to 2800, in fact he took the controls without saying anything. I meekly suggested that I thought the controller was supposed to query our altitude, not assume the Mode C is correct, and that our altimeter was fine. But he said he was more concerned about avoiding a bust. Later when we were cleared for the approach to home base and told to "maintain 3000 until established", I dutifully held us at 2800 instead. Okay, the MSA is 2700 so no obstacle clearance issues, but still... that did not sit well with me.

Clearly our Mode C encoder is out of, or nearly out of tolerance and needs checking. But what's the best way to handle this kind of situation in the air? My understanding is that ATC is supposed to say "say altitude", which my CFII effectively answered without being asked by saying that our altimeter read 3000. So isn't the worst that ATC could have done to tell us to "stop squawk altitude" and/or get our Mode C fixed? I can't help thinking that by taking us down to where we were squawking 3000 we were making it look even more as if an altitude deviation had occurred. Or could they still bust him for flying with out of tolerance Mode C?

BTW our pressure altimeter had been recently overhauled and our home base has an AWOS so we knew for certain that our altimeter was within tolerance, and we were keeping the altimeter setting updated as reported by ATC.

 

[dmccormack]

If it was checked and passed the test, tell ATC the plane is current and within limits and don't let him rattle your cage.

If it wasn't checked and you flew the same plane and got the same results your decision making skills need a reboot.

Precisely right on both counts.

 

[azure]

I am assuming this "episode" is using the same plane?

Yes.

If so then why didn't the owner get the encoder checked?

That's one of my complaints about this club, that squawks take forever to get fixed. But let's be reasonable here, this IS a fairly low-priority squawk. It is supposed to be checked out today.

If it was checked and passed the test, tell ATC the plane is current and within limits and don't let him rattle your cage.

The real question is, why is this controller trying to "rattle our cage"?

If it wasn't checked and you flew the same plane and got the same results your decision making skills need a reboot. IMHO YMMV.

Okay, this is the part I don't understand. You're saying I should refuse to fly this plane, because of an encoder that appears to be somewhat off but still within tolerance, because of one controller (I'm guessing it was the same controller both times, since it was the same facility and exactly the same MO) who keeps making an issue of it?

 

[poadeleted20]

The more you post, Liz, the more I question your instructor's competence. You deserve better quality training than you're getting from this person.

 

[dmccormack]

That's one of my complaints about this club, that squawks take forever to get fixed. But let's be reasonable here, this IS a fairly low-priority squawk. It is supposed to be checked out today.

The real question is, why is this controller trying to "rattle our cage"?

Okay, this is the part I don't understand. You're saying I should refuse to fly this plane, because of an encoder that appears to be somewhat off but still within tolerance, because of one controller (I'm guessing it was the same controller both times, since it was the same facility and exactly the same MO) who keeps making an issue of it?

Devil's Advocate: If you were actual, and admitted there was a Mode C problem, how does the controller know it will continue to indicate only 170' high?

FWIW, I had a Transponder burn up while IMC in Potomac airspace (they're a bit anal there). For a while before it died it was indicating all sorts of altitudes -- including 300' MSL where the terrain was 1100' and up.

They had me turn off Mode C, and when it finally died (Narco -- burned up), I remained IFR/IMC and simply reported where I was until handoff to CKB App.

 

[SkyHog]

I guess I'm just looking for advice on how to handle this situation in the future.

I know you don't want to hear it, but after reading your posts regarding your training, you need a new CFI. This guy is a joke, and he's going to kill somebody. Don't let it be you.

 

[poadeleted20]

Okay, this is the part I don't understand. You're saying I should refuse to fly this plane, because of an encoder that appears to be somewhat off but still within tolerance, because of one controller (I'm guessing it was the same controller both times, since it was the same facility and exactly the same MO) who keeps making an issue of it?

I'm not sure he's saying that, but I'm not. I can't count the number of times I've had one controller say my Mode C is reading high or low, and then the next one says it's fine. 150 feet off is within tolerance, and once you tell the controller your true altitude, that should end the discussion. As stated above, if a controller says your Mode C is 150 high, you do not repeat not fly 150 low to compensate. You press on flying on your altimeter indications, and if the controller doesn't like it, s/he can have you secure your Mode C.

 

[dmccormack]

The real question is, why is this controller trying to "rattle our cage"?

After a while, certain controllers know certain CFIs by voice.

It's not always good.

 

[azure]

What discrepancy? A Mode C indication of 3200' at the assigned altitude of 3000' MSL while on the correct altimeter setting is within limits.

Well, that's a point. The only time I've ever been instructed to "say altitude" was when flying a 172 with a Garmin x-ponder that was giving totally erratic readings, thousands of feet off. And that time they told me to shut off my Mode C too, as they should have.

I've many times, with this airplane, been asked to verify an altitude that was 100-200 feet higher than my indicated altitude. I've always given them my indicated altitude, and altimeter setting if it's been a while since it was assigned, and that's always been the end of it.

Except in this TRACON, and only on the two occasions I've posted about.

 

[azure]

Devil's Advocate: If you were actual, and admitted there was a Mode C problem, how does the controller know it will continue to indicate only 170' high?

Yeah, that could be a reason to make sure we know there's a potential problem with it. But this guy's MO (assuming it's the same controller) is to imply that we're causing a separation problem, NOT to tell us that we need to get our Mode C fixed. What you say about controllers recognising a CFI's voice is a little scary and makes me wonder if that isn't what's going on here. I won't post any details in public, but he's a self-employed professional CFI, not a part-timer, and pilot deviations due to busting airspace are one of his "hot buttons".

Ron, yes, thank you. That's exactly what I thought. It should be the end of it, and we should just stay on altitude. There shouldn't be an issue.

 

[roncachamp]

I am assuming this "episode" is using the same plane?
If so then why didn't the owner get the encoder checked?

Why would he? Wasn't the encoder within limits both times?

 

[roncachamp]

Devil's Advocate: If you were actual, and admitted there was a Mode C problem, how does the controller know it will continue to indicate only 170' high?

So what's the problem with Mode C that indicates 170' high?

 

[Dr. O]

File ASRS report...
Otherwise consider it just part of the learning curve... I often learn far more about 'what not to do' by watching others, than what to do...
Yeah, he saw his CFI going away in a flash in front of his eyes and over reacted... He could have simply asked the controller what 'he' wanted you fellas to do...
A simple question and the controller would then be in the position of giving you just-another-instruction to follow - which puts the birdie back on his side of the net...



Simply ask the controller what he wants you to do - don't worry, be happy...

denny-o

 

[dmccormack]

...the controller started started scolding us because our Mode C was showing us at 3200...

I typed 170 -- the original quote was 150