From aviation pundits with deadlines to meet and empty pages to fill, we hear a lot about the dangers of losing a vacuum pump (and consequently, attitude indicator and directional gyro) in IMC, and why IFR pilots need to (a) practice partial panel flight a lot, and (b) have a backup vacuum pump or electric AI.

As I’ve mentioned before, while it’s possible to find a couple of fatal accidents every year caused by loss of the vacuum pump in planes with retractable gear during a (legal) IFR flight, it is extremely difficult to find any in planes with fixed gear. In fact, I had failed to find any at all during my initial research. So thanks to Paul (N9002F) for drawing my attention to two from the NTSB files:

MIA98FA045

A Cessna 172 near Raleigh-Durham, NC on Christmas Eve, 1997

ATL91FA067

A Piper Cherokee near Hamilton, NC on March 18, 1991

A full report is available for the Raleigh-Durham crash, while only a summary is available for the Hamilton crash, but they both make interesting reading. To start with, as a brutal irony, both planes were equipped with functioning backup vacuum pumps, precisely what’s supposed to prevent this kind of accident. Even more ironically, it looks like the backup pumps themselves might have contributed to the accidents, at least in a small way.

Too much diagnosis, not enough flying?

In the first accident, the problem took place right after takeoff into low IMC. The pilot diagnosed the problem immediately and reported a vacuum failure to ATC, and then (as the NTSB determined from the wreckage) selected his standby pump. After that, the plane continued in a turn until it hit the ground.

The NTSB found that both the main and standby vacuum pumps were actually working and the gyros were undamaged — in fact, it is most likely that there was no failure at all. They also tested and discounted the possibility that a tube might have worked loose in flight, causing a (false) vacuum warning light on the panel. Furthermore, the flight lasted only 2 1/2 minutes, while it would have taken about 10 minutes for the gyros actually to spin down after a failure.

Why did the pilot report a vacuum failure? Could it simply have been a case of the spatial disorientation (his body disagreeing with the instruments), followed by the distraction of trying to troubleshoot a non-existant vacuum problem and select a standby pump? In my experience hand-flying my Warrior in IMC, initial climbout is by far the most difficult part of IFR flight, since the plane naturally wants to turn, and you have to keep strong rudder pressure to stay on course. Even the slightest distraction, like a radio call, and throw your course off 10-15 degrees if you’re not careful or a bit out of practice — in this case there was a lot more distraction than that, and sadly, having a backup vacuum pump to fiddle with (instead of flying the plane to a safe altitude first) probably made things worse.

Backup is not primary

The Hamilton, NC report does not give as much information as the other one, but it does mention the following:

• the pilot reported a pitot-static failure as well as a vacuum failure
• the pilot decided to continue the flight using the backup vacuum pump (and, presumably, no altimeter, VSI, or airspeed indication!!!)

After 28 minutes of erratic flying, the pilot finally lost control of the aircraft. The brief report does not indicate whether the backup vacuum pump also failed, but it does mention “improper use of equipment that affected the operation of the standby vacuum pump”. Did having a backup vacuum pump give the pilot the confidence to continue the flight?

Tentative conclusions

So neither of these is a clear case of a pilot losing control of a fixed-gear plane because of a vacuum pump failure. In the first case, both the primary and backup vacuum pumps (as well as the gyros) were all working properly, and we cannot know why the pilot thought otherwise; in the second case, the pilot was also facing a pitot-static failure, knocking out most of the panel, but decided to keep on flying for a half hour in IMC.

Are fixed-gear planes just a lot easier to fly partial panel, then? That’s what a 2002 ASF/FAA study suggests. Two groups of pilots were tested in actual aircraft rigged up to allow unannounced gyro failures — one group was tested in a Beech Bonanza (retractable), and the second group was tested in a Piper Archer (fixed gear). The results? The Archer pilots did a lousy job diagnosing the problem — it took them nearly 7 minutes on average to realize that something was wrong — but every single one kept control of the aircraft. The Bonanza pilots, presumably a much more experienced group of complex-aircraft pilots, diagnosed the problem faster — in less than four minutes, on average — but couldn’t all control their planes flying partial panel, and 4 out of 16 were judged to have crashed (i.e. someone without a hood had to take the controls).

I have two tentative conclusions from all of this, though I am far from an expert:

1. While installing an aftermarket backup vacuum pump in a fixed-gear plane might not hurt, it probably won’t help either, and might even be a dangerous distraction — investing the same amount of money in recurrent training, better maintenance, etc. probably makes more sense.
2. It’s a lot easier to maintain control of a fixed-gear plane flying partial panel, so upon losing gyros or the vacuum pump in a retractable, maybe checklist item #1 should be drop the gear to add drag and make the plane more controllable (if you’re above gear-extension speed, let that be the mechanic’s problem after you land). After all, the 25% fatality rate for retracts in the ASF/FAA simulation makes for lousy odds.

I’ll look forward to comments from people with other interpretations and suggestions, especially since I fly only fixed gear myself.