Vacuum system failures and the consequent loss of attitude and directional gyros has been the subject of two recent AVweb articles. In light of recent, high-profile accidents stemming from gyro system problems, those articles discussed how and why vacuum systems fail and the various options for back-up systems. But how does a pilot recognize a system failure? What if you don't have that back-up system installed yet? AVweb's Linda Pendleton offers some valuable tips on how to diagnose and understand when your gyros are lying, what to do about it and why you — yes, you — need to get more training.
March 26, 2001
|About the Author ...
Linda D. Pendleton is Manager
of Computer Graphics and Animation for
She is also the author of a book, Flying Jets, and scriptwriter for
several of the training videotapes published by King Schools, including
"Navigation from A to Z," "METAR/TAF Made Easy," and "Handling Emergencies."
Linda is an ATP with Citation 500 and Learjet type ratings, and a CFI with
airplane, instrument and multiengine ratings. In her 10,000+ hours of flight
experience, she's flown US Mail, freight, corporate, charter, commuter, and
served as an FAA-designated examiner for the Citation 500.
was always one of my best students. We called Joe (not the same Joe I was
flying with when we had the pitot static system
failure) "NightFlyer" because he worked as a carpenter all day
and usually came out in the evenings to fly. At any rate, he kept me on my
toes and made me a lot better instructor because I had to work so hard to keep
ahead of him and keep him challenged.
One Saturday afternoon Joe was on the schedule for a rare daytime flight
and scheduled with me for an instrument competency check. He did this
regularly and he was usually more than competent so it was always a challenge
for me to find a challenge for him. I decided that this time we would conduct
the whole flight partial panel. Of course, I didn't alert Joe to this before
takeoff. He would just have told me that if he knew he was going to have a
vacuum failure actual IFR, he wouldn't take off. And he would have been right,
so I just kept this little tidbit of information to myself.
As we climbed through 400 AGL, I slapped the hood down on Joe's head and
slapped pasties on the attitude indicator and heading indicator. Joe began
complaining mightily. I told him to continue. This went on for the whole time
we were airborne and for most of the taxi back to the tie down and postflight.
"After all," I told him, "you are paying me to make sure you
are proficient and current and ready for any eventuality. I am just doing my
job." Joe muttered something about perfectionists and we went out for a
days later NightFlyer booked a Cherokee for a cross-country flight and,
although the weather was Chicago's typical 800 and 1, he decided to go
after all, the tops were low and the freezing level high and Joe was competent
on the gauges. About a half hour after he left, I noticed Joe taxiing back to
the tie-down area. I went back to the student I was with and we continued to
discuss his cross-country planning until Joe burst in the door and rushed over
to give me a big hug. Well, I've had students greet me in various ways, but
this was not the usual so I asked Joe what was up. He told me that just as he
had climbed into the clouds the vacuum pump failed and the attitude indicator
had rolled over and died. At which point he immediately thought of all the
partial panel work we had recently done. He felt totally prepared for the
emergency and swore he would never complain about my being hard on him again!
(I rather suspect that since Joe is now an instructor that a fair measure of
his students receive partial panel checks. And by the way, his vow never to
complain again was soon broken.)
Many Ways to Fail
Joe's failure was a total vacuum system failure caused by shearing of the
shaft of the vacuum pump. Although this is a very common failure, it is not by
any means the only way the vacuum system can let you down. (See Scott Purdy's
excellent article on failure of the vacuum system
components.) However, beyond the vacuum pumps and associated plumbing,
there are the instruments themselves.
Why All Those Moving Parts...?
It is all Jimmy Doolittle's fault! Most of America knows General James
Doolittle because of his raid on Tokyo in the Mitchell Bombers, but as pilots,
we stare at his legacy every time we get in the cockpit. (Well, at least most
of us do.) On September 24, 1929, he proved that it was possible to fly solely
by reference to instruments using a Sperry gyro-horizon and a directional
gyro. Since then, we've been using gyro-stabilized instruments to keep the
shiny side up. (Okay, some of the fancy heavy iron out there use laser-ring
"gyros" with no moving parts, but, for the most part, those have not
filtered down to the GA fleet yet.)
Gyroscopic flight instruments work on the principle of gyroscopic rigidity
in space. A gyroscope will tend to remain in its original orientation in space
and will resist attempts to change its position. In fact, the gyros remain
rigid in space and the airplane rotates around them. Since typical gyros spin
from 8,000 to 18,000 RPM there is the potential for a lot of wear on bearings.
Without going into the detailed workings of each of the gyros (I want you to
be able to successfully deal with their in-flight failure, not rebuild them)
there is a high risk of failure with such fast-moving parts.
Aside from a vacuum pump failure, the most common ways you will see
problems with your vacuum system and/or gyro instruments are the
all-too-common directional gyro that precesses excessively or the attitude
indicator that shows a constant slight bank when the wings are actually level.
Those failures are not a serious problem when you can see out the window, but
in the clouds, the least they do is add tremendously to the workload.
...I Really Can't Take All The Credit
Although Joe was grateful for the training he received, the kudos go to him
for properly and promptly recognizing the problem and then dealing with it
before it put him into a life-threatening situation. Vacuum system failures
are not the cause of aircraft accidents. Non-recognition of the problem is the
major cause of the accidents. On the other hand, perhaps the pilot recognizes
that a problem exists but misdiagnoses the true nature of the malfunction.
The training that Joe received was good ... as far as it went, but let's
face it, no pilot in recorded history has crashed because someone reached over
and covered one of the flight instruments! At that point, the hard part is
over, the diagnosis is made, and the only job left is keeping the shiny side
up and the pointy end forward until a landing can be made. Now, don't get me
wrong, we all need practice in flying on partial panel instrumentation, but
additional training and practice is needed to get to that point successfully.
What's In A Scan?
Recent accidents have been blamed on vacuum or gyro system failure, but it
can't be that all those pilots were incapable of flying partial panel. When
you come right down to it, it's not that difficult after all, there are
fewer instruments to scan! So, what's happening to all these folks?
instrument scan is something we tend not to give much thought to once it is
mastered, but let's take a closer look at what we are actually doing during
the scan. It's impossible to quickly interpret the readings of all the gauges.
What we're really doing is trying to identify the gauge or gauges that are not
showing what we want to see. If you're aim is to maintain an altitude of 4,000
feet, the only time you should take conscious notice of the altimeter is when
it reads something other than 4,000. If you're maintaining a heading of 120
then the only time the heading indicator should capture your attention is when
is say something other than 120. And that's when the problems begin.
If the heading indicator is drifting off our desired 120 heading, what's
wrong? Are we simply rusty on the gauges and unable to accurately hold a
heading? Or has the heading indicator failed? Or, is it the attitude
indicator? Or perhaps the whole vacuum system has gone south. If the correct
answer is not determined quickly, the success of the outcome will be seriously
in doubt. Quick diagnosis is the key.
How To Decide What Quit...
So how do we decide what the problem is while the airplane is still
right-side up and level? Remember your instrument instructor's warning against
fixating on any one instrument? When your scan picks out an instrument you
think is wrong, the urge is to stare at that gauge and try to fix its problem.
Truth is, the gauge that does not match the rest of your instruments may not
be the problem. If your heading is drifting, it may be a bad heading
indicator, or it may be the fault of an attitude indicator that is either at a
perpetual bank in level flight or in the process of failing or it may be a
failure of the vacuum system itself. How do you decide?
Check the other instruments. If the attitude indicator shows wings level,
what does the turn coordinator show? What does the compass show? Verify the
readings on your vacuum instruments against each other and against the pitot/static
instruments and the compass. Make very small changes and only make one change
at a time and verify that the anticipated reaction shows up on the
instruments. Once you determine the bad guy, get it out of your scan. I carry
instrument covers in my flight kit all the time. Folks have kidded me that I'm
the complete flight instructor ready to inflict partial panel on an
unsuspecting student at any time. Not the truth. I carry the covers so I can
cover a failed instrument and get it out of my scan. Remember, your eyes will
be drawn repeatedly to a failed instrument and this will destroy your scan.
Eliminate it and the distraction it causes.
...What Training Is Needed?
can we train to quickly recognize and recover from instrument and/or vacuum
system failures? Unfortunately, the training required is not readily available
to many pilots and it doesn't take place in an airplane. Partial-panel
training is good and valuable and should be practiced by everyone
periodically, but it only addresses how to keep the airplane upright once a
problem is diagnosed. Unfortunately, when the instructor reaches over and
covers an instrument, that takes care of the diagnosis part of the exercise.
What's necessary is training to diagnose an attitude indicator that's
slowly winding down or a heading indicator that is precessing abnormally. That
kind of training requires a flight-training device. (The generic word used is
simulator, but a simulator is a device that accurately represents a particular
make and model of aircraft.) Your first session in a training device with the
ability to simulate various instrument system malfunctions will be a
revelation. Most pilots I have observed undergoing this type of training have
lost control of the aircraft at least once before the situation was properly
diagnosed and dealt with. Once you undergo this type training yourself you
will have adequate motivation to maintain currency in these vital skills.
So what's a pilot to do? Obviously, getting some "simulator"
training is the best course of action. If that kind of training is not readily
available to you, there are some things you can do until you can manage to get
the training. (I don't consider this a choice. If you intend to fly in the
clouds with any regularity, you NEED this training. It's not a "nice to
have" situation it's a life and death necessity.)
Some Do's and Don'ts
develop a comprehensive understanding of how all the flight instruments
operate and their most common failure modes.
DO get regular partial panel practice. The more practiced you are
the more second-nature this task will be. Remember that when the time
comes you won't need any more tension in the cockpit.
DO carry instrument covers to immediately remove a failed gauge
from your scan.
DO develop a plan for sorting out a failure. For example, how
will you determine whether you have a failed attitude indicator or a
failed pitot/static system? Well, how about trying a slight pitch up. If
the attitude indicator moves and none of the pitot/static instruments
respond appropriately, it's probably a pitot/static failure. If, however,
the attitude indicator doesn't move but the altimeter, airspeed, and VSI
respond appropriately, slap that sticky on the AI! Do these "what
if" exercises before the real need arises. Do these "what
if" exercises while they are still exercises!
DO tell ATC about your problem as soon as you have things under
control. They can't fix the problem, but they can do things to make your
DO get training in no-gyro approaches. Although you're going to
want to get to VFR weather if possible, you may have to land in instrument
conditions and practice with no-gyro vectors will come in handy. You will
probably find ATC happy to help you practice this since they need practice
also to remain proficient. Just don't ask during rush hour at O'Hare or
DO use all resources at your command. A savvy passenger in the
right seat can read checklists and charts and help you with your scan.
DON'T get involved with trying to diagnose WHY something failed.
Just determine that it failed and keep the airplane under control. After
all, you can fix precious little while flying the airplane, so do your job
and resign to pay the mechanic once you land safely.
DON'T forget your prime responsibility control of the
We have to change the way we train instrument pilots
and PCATDs may be an answer to the problem. Any system that will allow
instructors to introduce subtle instrument failures will be an improvement. As
the regs stand now, up to 10 hours in a PCATD may be credited toward an
instrument rating. The regs should also require that all time spent in a PCATD
or other training device should be spent simulating those failures that cannot
be demonstrated in an aircraft in flight. Until that time, don't put off
practicing your own "what if" scenarios. If you thoroughly think
through all possible failures ahead of time, you'll be in a far better
position to recognize and deal with them when the time comes.