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I suffered my first actual in-flight aircraft failure yesterday.… - delta_november

Jun. 30th, 2014

05:51 pm

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I suffered my first actual in-flight aircraft failure yesterday. The setup couldn't have been better.  I was taking a nervous family member for a first flight.  Concern had already been expressed during the climb out about the flashing red light on the transponder -- did this indicate something bad?  Reassurances were made that this was a good sign -- ATC can see us on their radar.  Then, half an hour in, the "Low Vac" light starts blinking red.  This does indicate something bad.

At this point, some background may be useful.  Most small aeroplanes have three gyroscopes to help keep the pilots oriented.  The Turn Coordinator lives in the instrument panel, and is electrically powered.  It is a rate gyro, telling you the yaw rate in degrees/second.  The Heading Indicator tells you which way is north.  In my plane the heading gyro lives in the tail and is electrically powered.  It sends pulses to a remote display on the instrument panel.  Finally, the Attitude Indicator tells you which way is up.  Also known as an artificial horizon, it lives in the instrument panel an is vacuum powered.  Air enters the device, spins some little paddlewheels, and then runs out a hose towards the vacuum source.

The vacuum comes from a rotary vane pump spun by the engine.  The vanes are made of graphite, and self-lubricate as they slide in and out each revolution.  Sooner or later they break and jam.  The drive shaft, designed for this eventuality, snaps to prevent further carnage.  I'm now told that it is recommended to replace the vacuum pump every 500 hours of flight.  I have to say I'm not impressed by this engineering at all.  Sure, it may be a "simple" system, but it's still terribly unreliable.

I was very fortunate, for a number of reasons:

  1. It was daytime, and I was in Visual Meteorological Conditions.  I could tell which way was up by looking out the window, so I had no pressing need for the attitude indicator.  If circumstances had been different it would have been sorely missed.

  2. My Mooney has a flashing red low vacuum light which immediately caught my attention.  I knew about the fault before the gyro had time to spin down.  On the Cessnas I trained on there is only an analog vacuum pressure gauge which is not frequently checked.

  3. I have something that I can do about a vacuum pump failure.  My plane is fitted with a standby vacuum system, which connects engine manifold to the instrument vacuum line.  This will keep the gyro spinning, provided the throttle is pulled back to reduce the manifold pressure.  On the training Cessnas there would be no solution but to proceed on partial panel.

In the event I was able to calmly pull the standby vacuum control and extinguish the flashing red light before my passenger was any the wiser.  Return home was uneventful.

Some time this week I'll take the plane to my mechanic and see what the damage is.  Has the pump sent graphite shards into my attitude indicator?  Do I have any inline filters that may have protected me?  I'm comfortable noodling around Southern Ontario on the standby vacuum, but want this fixed before I go further.


[User Picture]
Date:July 3rd, 2014 11:57 am (UTC)
Interesting...what's the procedure if you lose the artificial horizon while flying under IFR?

(Glad you are OK, obviously, and hopefully the red flashing light has not deterred your nervous passenger from future flights!)
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[User Picture]
Date:July 3rd, 2014 01:42 pm (UTC)
Your ground school books should have a short section on "partial panel" flying. Essentially you can use your remaining 4 instruments to figure out the attitude.

You get your pitch clues primarily from the Air Speed Indicator (ASI), with confirmation from the altimeter (ALT) and Vertical Speed Indicator (VSI). Your Turn Coordinator (TC) gives you your yaw and roll rate, which will tell you about your roll attitude.

It's remarkably similar to flying an aircraft VFR in a climb. When you do your first takeoff in a C150 (soon I hope!) you will be pitched up and will probably not be able to see the ground over the nose cowling. Instead, you will hold your air speed at 68 KIAS. If the ASI reads too low, then push forward on the wheel. If it reads too high, pull back. This puts you at your very best climb performance. The actual pitch angle isn't so important, and will change somewhat based on air temperature and plane gross weight.

Once you have climbed to 500' ASL you will start the cross-wind turn of your circuit. You don't want to turn too fast in a climb, because that's asking for a spin which is deadly at low altitude. You'll aim for a "rate one" turn, which is 1 minute for 180 degrees, or 3 deg/sec. You'll read this from the TC.

So in a VFR climb you'll pay a lot of attention to the left-most instruments in your six-pack. You'll also look at the ALT to see when you're at the altitude you want, the Heading Indicator (HI) to see when you're done your turn, and often the VSI to either laud or grumble about the climb performance of the aircraft. The AI gets very little attention VFR. You're also looking out the window a lot to make sure you don't run into any other planes.

The key, of course, is that if you get completely disoriented in a VFR climb you can just push the nose down and see the horizon. In IMC you can't, and flying partial panel you must be able to not only cruise, climb and descend but also recover from stalls, spins and spirals.
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