23 July 2008

Celebrate 23 July

Today is the 25th anniversary of the famous (in aviation circles, anyway) Gimli Glider incident, the only aviation accident known to be caused by the metric system. Well, partially. I'll tell you now, no one was seriously hurt in this accident, so you can read this as it's intended: for comedy.

The Gimli Glider was a Boeing 767, which departed Montreal on 23 July, 1983, and was supposed to fly to Alberta. But there was a snafu: the fuel gauges weren't working properly, and the flight crew had to go on a dripstick measurement (not really; if they'd pulled a circuit breaker, they'd have seen the proper fuel reading and fixed the problem, but the notes on the matter were unclear), which was made in metric. But Canada had only recently switched to metric and it was common practice among pilots and maintenance personnel to rough-estimate the metric measurements in "real" measurements, a conversion that got fudged that day by multiple people, making it appear that there was more fuel on board than there actually was. Consequently the aircraft took off with far too little fuel to make it all the way to Alberta. They ran out of gas over Ontario and both engines shut down, thus turning the normally well-powered 767 into a wide-bodied glider with beverage service.

But there were more snags. The aircraft, which was so new the seats were still comfortable, had one of those new-fangled electronic flight systems, wherein most of the flight instruments rely on power supplied by the aircraft engines to operate. So once the engines went out, most of the flight instruments did as well and the pilots had to go to backup instruments, among which was not included a vertical speed indicator, which might have been handy to tell them how far they were going to make it before they hit the ground and to help select a best glide airspeed (best-glide not being mentioned in the manual, apparently, as the pilot had to guess at it with a mental wag, which he only knew how to do because he flew gliders recreationally). Not only that, but the aircraft hydraulic system, which moved the flight control surfaces and lowered the flaps and opened the gear doors, was pressurized by the engines. The backup system (at least there was one) involved a small ram air turbine which would drop into the airstream beneath the body of the jet and relied on the aircraft's forward motion to pressurize the hydraulics. Of course, once an airplane becomes a glider it tends to slow down, and at landing it slows down a lot; less forward motion means less hydraulic pressure, and just when you need it most (you want to lower the flaps and landing gear as you slow down and approach the land, and when you're slower you need larger control-surface movements to control the aircraft.)

This comedy of errors finally ended at Gimli, Manitoba (home of the Crown Royal whiskey distillery, so you know which drink to serve at your Gimli Glider party), where the aircraft's first officer had been stationed in the Canadian Air Force. The air force base, however, had been decommissioned, and the runway turned into a drag strip, which at that very moment was hosting a "family day" auto racing event.

The pilots tried to drop the landing gear using gravity (saving limited hydraulic pressure to lower flaps and, you know, fly the damn thing), but while the gear came down the nosewheel failed to lock into place. With the gear down the aircraft slowed dramatically, making it much harder to control, and in order to lose altitude to make the landing (cars and campers having been herded off the runway by local fire and police) the pilot had to slip the aircraft, something you pretty much never do with a plane that big (slip means "make really unaerodynamic so it'll drop out of the sky" and it involves putting the rudder and ailerons at cross-purposes, something that probably required both pilots to have hands on the controls because there was so little pressure to move them with); as the pilots drove into the airfield at very low altitude one passenger reported that he could tell what clubs people were using on the golf course they were flying over.

They did land on the runway, though, which is pretty damn remarkable in and of itself and although the initial problem—running out of fuel—is certainly partially the fault of the crew, they did a good job in dealing with the issue. The captain stood on the brakes—probably literally stood on them since the brakes use hydraulic pressure, too, and there wasn't much of that left—and blew out two tires, and the nosewheel collapsed (it hadn't locked, remember), so the airplane skidded down the runway on its nose and veered into a guardrail. That was actually a good thing; the guardrail absorbed some speed and the airplane stayed on the runway.

There were no injuries to any of the passengers or aircrew during the landing, so bravo to the pilots for that. However, several passengers were injured evacuating from the airplane; they used the slides that go out the rear door, but since the plane was resting on its nose the slides didn't reach all the way to the ground. Fortunately it was little more than scrapes and bruises and everybody was treated at the scene.

So that's the story of the Gimli Glider. The aircraft in question was fixed up and reentered service, flying several thousand hours before retiring in 1999 (and I bet only a few flight crews ever told the passengers they were on the famous Gimli Glider). It's worth pointing out that this was the first significant in-air incident for the 767, occurring just about a year after the aircraft entered service. The hydraulic systems have been upgraded fleetwide, so you needn't worry the next time you board one; should your 767 run out of fuel (if the U.S. adopts the metric system, for example) your glider will have all the controllability it needs to execute a safe landing. And with any luck you'll land near a whiskey distillery.

1 comment:

Ayzair said...

Nope, sorry, wrong celebration! July 23 marks that hallowed occasion 31 years ago when one of the most fabulous women ever was born!

Geez, 31. That's old. That's actually in my 30s, not just 30. Yuck.