Instrument failures

[Evo]

I'm at the partial/limited panel stage of the IMC, which is making me wonder about what instrument failures are likely in real life (as opposed to an instructor with too many post-it notes :) ). So far I've seen two failures, and both times it was the DI (once on a PPL navex, the other time en-route to Duxford with Genghis). In VMC it's not too hard to deal with once you realize that there's a problem. In IMC ... hmm, not too much of an issue if you're tracking a navaid, but could be a pain otherwise.

I'm guessing that in a spamcan the most likely is the limited-panel scenario of a vac pump failure leading to the loss of the AI and DI, along with a blocked pitot tube leading to an incorrectly-reading ASI. Any thoughts...?

[FlyingForFun]

I've only ever suffered instrument failures in VMC (DI on two aircraft, and AI on one). I suspect the most difficult thing in IMC would be recognising that an instrument has failed - with no instructor there to tell you it's failed, you'd have to realise that it wasn't matching other instruments when you cross-checked it.

The second hardest thing would be ignoring the failed instrument. For the first few hours after my AI failed I felt very disoriented, even in VMC, looking at it showing me something which didn't fit what was going on outside. I've heard someone suggest that if you do any serious IFR flying you should buy some suction caps like the ones your instructor probably uses, so that if an instrument really does fail you can cover it up - sounds like good advice to me!

FFF
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[Fly Stimulator]

The only one I've had for real was a vacuum pump failure in a PA28 while I was doing my night training last winter.

We noticed the suction failure, and that we'd lost the AI and DI, but fortunately it wasn't too hard to deal with since it was a beautifully clear night and there were plenty of lights for reference between the IoW and Lydd.

I suspect the amusement value of the same thing happening in real IMC would be quite low :eek:

[ft]

Is the turn rate gyro required to have a different power source than the AI? If both decide to go on an early vacation in solid IMC... well, I’m not instrument rated by far. What do you do?

Cheers,
Fred

[Timothy]

Partial panel flying, both recognition and recovery, are major training topics. They should be trained by competent IMC or IR instructors.

Instrument failure in IMC is a real killer (the one that comes to mind was the light turboprop (B200 or Bandit, I forget?) out of LBA a couple of years ago.)

IMC and IR students should learn it and practice it. I would be very worried about someone trying to learn it from pprune, or even books.

But certainly high on the list of "immediate actions" is to cover the failed instrument, as it is very difficult to ignore the instrument that you have had drummed into you is your main source of reference.

I would suggest to anyone who is serious about IFR that having two AIs and either two suction sources or one of the AIs being electric is a very good idea. If that isn't possible, then certainly ensure that the turn co-ordinator is the opposite to the AI (ie one suction, one electric). I would refuse to go into IMC in an aircraft where both the AI and the TC are powered by a single suction source.

And having an autopilot that works off the TC (such as the S-TEC) is also potentially a life-saver.)

Instrument failure in IMC can kill very quickly indeed (seconds not minutes).

Sorry to sound like an old fart.

W

[ModernDinosaur]

It's perhaps worth noting that in some modern IFR aircraft, ALL of the primary gyro instruments (AI, DI and TC) are electric - there is NO vacuum system. This seems sensible to me as (in my admittedly very limited experience) it is usually the vacuum pump which fails.

In all-electric IFR aircraft there is usually a guarded switch which connects a battery inside the AI to keep this instrument running even in the event of a complete electrical failure. The battery will provide at least an hour of reliable AI operation. OK, so you're on the compass rather than the DI, but at least you know which side would be blue if it wasn't for that wretched cloud.

As always, it's important to know your specific aircraft rather than relying on generic rules of thumb, some of which are being overtaken by the latest advances in aircraft technology.

MD.

[RodgerF]

Quote:

We noticed the suction failure, and that we'd lost the AI and DI, but fortunately it wasn't too hard to deal with since it was a beautifully clear night and there were plenty of lights for reference between the IoW and Lydd.

Beware! Your instructor must have mentioned this, but bright lights on a clear night are a major source of spatial disorientation.


Quote:

Is the turn rate gyro required to have a different power source than the AI? If both decide to go on an early vacation in solid IMC... well, I’m not instrument rated by far. What do you do?


AFAIK it is a requirement of IFR certification that this is the case.

[Evo]

WCollins


Partial panel flying, both recognition and recovery, are major training topics. They should be trained by competent IMC or IR instructors.
<snip>
IMC and IR students should learn it and practice it. I would be very worried about someone trying to learn it from pprune, or even books.


Don't worry, i've got a very competent instructor. At the moment I'm fairly happy that provided I noticed and understood the failure I could remain in control of the aeroplane while I screamed at ATC for assistance, and then turn and/or descend out of trouble using the instruments that i've got left. I couldn't fly an approach limited panel, but i'm not going flying in weather when it's necessary. Need to do that, get an IR.

As for noticing the failure, well ... I hope so, but I'm not sure. I'm trying to learn to compare the instrument I'm looking at with what the others are telling me, but it's hard. How well I can do that may be the issue that decides how I use the rating - it seems daft to use it in anger if I may follow a dying AI into the ground.

As to learning partial/limited panel from PPRuNe, I'm not trying to do that. I'm just trying to get an idea if there are some instruments that are more unreliable than others. A total failure of the static pressure system seems fairly unlikely, for example (switch to alt. static, then break VSI glass - I've been reading Thom ;) ), whereas I've only got 80 hours and i've seen two DI failures.


Sorry to sound like an old fart.

Doesn't sound that way to me... :ok:

[Flyin'Dutch']

The test to see if people are competent in partial panel flying bears little resemblance to a real life failure.

Second, separately powered AI seems to be the way forward.

FD

[Keef]

What I liked about my US IFR instructor (and the syllabus) was the constant cross-check, and "primary-secondary" instruments for each action. As long as the pilot sticks to that while flying IFR, a failure will become obvious very quickly. I found it worked better for me than the "selective radial scan" I was taught for the IMC rating.

WC is right - the only way to learn it is to do it (often) in an aeroplane. I make a point of doing at least one IFR-IMC (or simulated) sector and approach a month. I'm not sure that's enough, either.

I recall that it's a requirement for IFR certification that the AI and the TC should be powered from different sources. Our AI is vacuum, the TC is electric. The TC also shows a red flag if the electricity is missing.

I have trouble remembering the compass timing for the rollout on partial panel - before North, after South, is it - or the other way round? I suppose that if I were doing it in anger, getting the roll-out right first time wouldn't be my highest priority.

[Timothy]

Keef

Don't try and use the compass to turn on. Do timed turns (based on rate 1 = 180deg/min) with sharp entry and exit, then let the compass settle down before correcting the last little bit.

W

[Evo]

Keef


I have trouble remembering the compass timing for the rollout on partial panel - before North, after South, is it - or the other way round? I suppose that if I were doing it in anger, getting the roll-out right first time wouldn't be my highest priority.


I remember the rather-dumb Never Near North, South Seen Soon (so before North, after South) - anybody got a better one? :)

As for acceleration errors, it's ANDS - Accelerate North Decelerate South...

but i've been taught to use timed turns as well, not that i'm any more accurate with them...

[FlyingForFun]

I remember the rather-dumb Never Near North, South Seen Soon (so before North, after South) - anybody got a better one?We all like to head towards the equator for the warmer weather. The compass does too - which is why, if you're turning towards the equator, it gets there early. If you turn towards the poles, where the weather is cold and nasty, the compass doesn't like that any more than we do, so it arrives late.

This works for me. The reason I like it so much is because it's the only one I've found which works in both hemispheres. Not that I've ever been flying in the southern hemisphere, but that didn't stop my being examined on it in the ATPL exams.

FFF
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[Genghis the Engineer]

What have I seen - well clearly the rather entertaining DI failure with Evo that was noticed when we worked out that we were pointing directly at Luton instead of Cranfield on the way back from the Duxford fly-in (a bit of muck in the hose I think, we later blew the system through and it's been fine ever since).

Several particularly untrustworthy compasses, usually in homebuilts. PFA and BMAA inspectors are not fond I think of carrying out compass swings.

Twice an ASI failure, once due to flight in icing conditions in an aircraft without a pitot heat, once was effectively a ground-fault, noticed immediately after landing - I flew a single circuit and landed without incident.

And I noticed a VOR yesterday reading about 4° out from what it should according to AIP at the location I was at, but that's probably just atmospherics and terrain since it is usually fine.

G

[A and C]

I had a vac pump fail when doing an IMC revision detail , the student did well to pick it up and revert to limited panel.

The learning exercise was compleated by Lyneham ATC with a "no gyro" PAR .

Good training and not a post it note in sight !.

[Saab Dastard]

EVO,

I've had a vacuum failure in a PA28 with AI and DI both U/S. T/C Electric so unaffected.

Fortunately I noticed it on the taxi checks and returned to the apron - not so easy to spot if it happens gradually in the air.

Not doing IMC yet, but still didn't fancy flying it even in VMC!

SD

[SlipSlider]

".....but still didn't fancy flying it even in VMC!"

SD, I'm intrigued, why ever not? For VMC there are simple alternatives to use instead of AI and DI ..... :confused:

[FlyingForFun]

For VMC there are simple alternatives to use instead of AI and DI ..... Yes, been there, done that.

AI - no problem. Although, as I said earlier in the thread, having a wrong AI can be disorientating until you're used to it.

DI, on the other hand, is something which I'd rather not fly too far without if I have choice. Not that I can't fly without it, but it does make it considerably more difficult. Especially in an aircraft where the compass is only intended to be usd occassionally to cross-check with the DI, and so it's not positioned anywhere that's easy to see. No problem at all in the local area when I know where I'm going, it's when you're doing a cross-country, maybe in an area where there aren't too many features, that it's nice to have your heading displayed in a handy format in front of you.

FFF
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[englishal]

I wouldn't fly without a servicable vaccum system if I noticed it before the flight, if in flight and a vacuum system failed and I was in VMC I would continue the flight, if there was a chance of becoming IFR, then I'd land ASAP. I think it is illegal to take off with a known deffective vacuum system and fly under IFR...it is in the states anyway....[common sense ?]

Cyer
EA:D

[Charlie Zulu]

Hi Evo,

I remember the rather-dumb Never Near North, South Seen Soon (so before North, after South) - anybody got a better one?

The one I was taught on my IR was "UNOS"... "Undershoot North, Overshoot South".

Accelerations errors was ANDS but you know that one already.

Seems to work for me!


WCollins / Keef,

For large turns I'd prefer to use Mag Compass turns, but for small turns, I just count them.... "1 thousand and 1, 2 thousand and 2, 3 thousand and 3"... should give me ten degrees or so.

Ah partial panel approaches / missed approaches / holds were fun!!!

Best wishes,

Charlie Zulu.

[Timothy]

Stopwatch works better.

W

[redsnail]

Yeah, had a couple of instrument failures along the way.
The usual vac pump failures in singles etc. No biggy, VMC.
Also used to happen in the Islander too. Was only permitted to continue flight (multi sector) if VMC was assured. I had to pull the little plastic drive out of it and then secure.
Only happened with dry vac pumps, never seen a wet vac pump fail.
Slave mechanism failures. Good idea to check it on the runway.
AHARS failure in the Dash 8. Absolutely nothing like the sim detail.
(basicly all the control instruments. Atitude, heading reference system). No way was I going in IMC with that one.
ASI over readings. Had to keep cross checking the captain's. Pain.

[In Altissimus]

I had a wierd one (to me anyway) a few weeks ago when, after being parked at a slightly 'unusual attitude' overnight in pouring rain, and after 1 normal short flight, the ASI and altimeters both failed at about 300' after take-off.

After the initial disbelief this resulted in a curt call back to the field - then blanking everything around me out and concentrating on doing a circuit pretty much by eye and tacho.

It was uneventful, although being fairly new to type and at an unfamiliar airifield didn't help.

One of the FI's at the club just sucked (gently!) on the static vent, spat out a lot of water, and assured me all would be OK - which it was.

Afterwards I just felt annoyed that this type of flying hadn't been covered in training...

[Tinstaafl]

I've had a vac. pump fail, losing my AI & DI, 2 x AI only, and 2xASI. The vac pump & 1 x AI were both at night in a part of Oz with stuff all ground lighting. The ASI and 1 x AI were daytime in IMC. It was a bug for one ASI, an automatic/airspeed deactivated pitot cover the other ASI failure and a stuffed bearing (I think) for the AI. The AI wouldn't sit straight ie it was at odds with the flight director command bars, the second AI & the performance instruments.

The vac failure & one of the sole AI failures were at night in a typically minimally equipped C172 ie single of each instrument. The single AI was during a student's Night VFR navex, the vac pump while I was doing the same student's Night VFR flight test. Unlucky ******! He passed, in part due to handling the failure so well.

If it can fail, it will. If you fly using mechanical instruments then sooner or later one will fail. It's no joke about needing decent lim.panel skills including lim. panel unusual attitude recovery. As was rightly said: Have with you something to cover the failed instrument(s), most importantly covering a failed AI.

Whatever you do, don't ever fly something in IMC that doesn't have two independent attitude references. Either two direct sources ie AIs with independant power sources, or a direct + an indirect ie AI + TC/ASI, also independent power sources.

[Fujiflyer]

Had an "interesting" experience last weekend (Monday 25th).

Departed Menorca (R01) with a clearance "not above 1000ft" consistant with the local controlled airspace (IFR). Menorca APP cleared me "stay below 1000ft, via point Echo (an easily identifiable point near to the Isles). A few minutes after takeoff the APP controller (obviously very concerned) ordered me to descend as he believed I was in the IFR class A airspace surrounding the region. His readout was reporting 2500ft :(

I assured him several times that I was definately at no more than 800ft agl, he eventually seemed to accept this although did continue to order me to descend. I could not have descended much further without unhappy consequences.

It turned out that my altitude encoder/transponder had failed. This was further verified by Perpignan APP - however their readout for me had reversed in error - I was at FL45 and their readout (secondary radar) was FL30. I flew the rest of the journey without mode C selected.

[Timothy]

I have owned my present Aztec for over eight years and Mode C has never been accurate despite thousands of pounds spent on labour and replacements.

W

[Fujiflyer]

WCollins, what did surprise me though was the controller's reluctance to accept that I was really only at 800ft as per the clearance. Over here (UK) they would always verify a mode C readout via the RT (ie check with the pilot).

I guess the MAH controller was just playing safe especially given the possibility of missunderstanding each others' accents. Still it was frustrating given that I had understood and readback (several times) the clearance, confirmed my actual alt on the qnh (which was 1012 so pretty close to FL pressure reference) yet was being ordered to descend. As I left the zone he was still reporting my recorded alt as 2500' (to IFR traffic) but stating that the pilot was saying he was at 800' only.

I am also surprised that the error (fault) had reversed in direction at Perpignan. Obviously there was a genuine problem with the Mode C.

Was all a bit of a shame really. I had a 800nm journey ahead and didn't need this at the very beginning. Wifey was getting "concerned" (she was listening to the RT) - I assured her that equipment failures were not a big issue in their own right - its how you handle them.

Rich