Most pilots are accustomed to an instructor’s simulating the failure of an instrument. This usually is done by covering the "ailing" gauge with a Post-It note or an object to which a small suction cup is attached. Most commonly "failed" are the airspeed indicator during visual flights and the gyroscopic instruments during instrument training flights.
It was, therefore, not surprising to notice the instrument instructor reaching across my hood-limited field of vision with a circular Post-It. But instead of covering the attitude indicator as I had expected, he covered the altimeter.
"Hey," I yelped. "Altimeters don’t fail. How am I supposed to make an instrument approach without an altimeter?"
"Well, this one failed," he said. "And don’t ask me what to do. You’re on your own."
Flying on instruments without current altitude information rapidly creates a feeling of helpless anxiety. It takes little time to appreciate that the altimeter is the only flight instrument for which there is no backup or redundancy during IFR conditions. Although generally regarded as bulletproof, it can fail and be the cause of great concern (or worse). This is why some countries require that all IFR-certified aircraft be equipped with dual altimeters and static sources.
If an altimeter fails during cruise, a pilot can be temporarily misled into believing that he is doing a masterful job of maintaining altitude. Should the altimeter hands stick in position, tap the glass face or rotate the altimeter-setting knob to see if they can be dislodged. If so, consider yourself lucky and have the instrument replaced before the next flight.
(If an altimeter failure is accompanied by what appears to be an inoperative vertical speed indicator, the problem is related to the static system and not the individual instruments. Select the alternate static source, if available.)
Another warning of impending failure is an altimeter-setting knob that becomes progressively more difficult to turn. This usually indicates that moisture has entered the instrument through the static system and has begun to corrode the mechanism.
The only actual altimeter failure I ever had was when a 10,000-foot pointer came loose, fell to the six o’clock position, and stayed there. This wasn’t a serious problem; knowing the altitude of a Cessna 172 within 10,000 feet didn’t take great skill or cunning. But a large hand coming loose when flying IFR can be very serious.
Never take an altimeter for granted. Any indication of abnormal operation should be corrected before the next flight.
So what is an instrument pilot to do when confronted by altimeter failure? If in VFR conditions at the time, he obviously should remain that way. If in cloud, he needs to make air traffic control aware of his emergency and request a clearance (and suitable altitude block) to an area where VFR conditions can be found.
Failure of an encoding altimeter usually prevents a transponder from providing ATC with altitude data. But if a controller can detect altitude changes, he could keep the pilot updated with current Mode C altitude indications. This information would not be sufficiently accurate to shoot an approach to minimums, but it would be useful enough to maintain a safe altitude.
Fortunately, independent blind encoders are rapidly replacing the more expensive encoding altimeters. If the aircraft has one of these, a controller can assist a pilot during an IFR approach to higher-than-normal minimums by constantly calling out aircraft altitude during descent. (Note that a Mode C transponder within limits is allowed to be in error by as much as 300 feet.)
With a Mode C altitude readout available on his radar screen, a controller also can assist a pilot in descending to a safe altitude for intercepting the glideslope of an ILS approach. Once established at the outer marker inbound, a pilot without an altimeter should be able to descend safely on the glideslope. He can determine when the aircraft has reached a safe decision altitude (DA) by using the middle marker as the missed-approach point. (Pray that it is operative.) Note that this is purely an emergency procedure.
If such assistance is not possible, a pilot might consider entering a holding pattern at the outer marker. He can then descend slowly (using the VSI for guidance) until the aircraft is slightly below the glideslope. At this point, altitude should be held approximately constant (again using the VSI) until intercepting the glideslope and beginning a normal descent.
Another alternative is to head for a military base and take advantage of the precision approach radar facility that usually is available there. During a PAR approach, the controller uses vertical-scanning radar to literally talk a pilot down to the runway threshold.
Other emergency tactics include depressurizing a pressurized cabin (when practical) and using the cabin altimeter as a conventional, less sensitive altimeter. Also, when the throttle is set to a fixed position (by using the friction lock), a manifold pressure gauge can be used as a very crude altimeter by recalling that manifold pressure will increase one inch during each 1,000-foot loss of altitude. The altitude readout of a GPS receiver, however, would normally be more accurate.
There is no satisfactory backup for an altimeter. Pray that it never fails you.
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