Instrument failures
 

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...?

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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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:

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

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

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.

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.

WCollins

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.

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

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

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.

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

Keef

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...

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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Instrument failures I've seen....
 

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

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 !.

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

".....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:

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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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

Hi Evo,

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.