Software Musings

PDM at its Worst

Posted on: 21/01/2007

I teach various classes on Pilot Decision Making (PDM to aficionados) and still find myself bemused and concerned about a decision I made last weekend.

As we do on many weekends, a group of us flew half a dozen aircraft from Rockcliffe in Ottawa to LaChute in Montreal for lunch. As the others were leaving Rockcliffe, I found I needed a litre of oil and had to walk back to the club house to buy one. This meant that I left perhaps 10 minutes after the others and, since I have the slowest aircraft, I arrived in the LaChute area when the others were already sitting down and eating. The aircraft in front of me in the circuit (not one from our group) had a problem on the runway and the runway was closed for 15 or 20 minutes, during which time I slowly circled to the south of the field. The runway was then cleared and I landed without problem. I’m only recording this to set up the situation for what happened as I departed.

I had my lunch and we all got up to leave. Although we were now running later than normal, I was under no time pressure—I was flying alone and, since my wife was away in the UK, there was no hurry to get back. I started the engine and did my engine runup checks. During these I heard a strange tapping noise somewhere behind me. I ignored it, assuming that it was something on the back seat resonating with the aircraft’s vibrations. I then noticed that one of our group was having trouble starting his aircraft and so closed my engine down to see whether he needed help (or a lift back to Rockcliffe). He didn’t and so I started up again and did a second engine runup. Again the strange noise.

Ignoring the noise I taxied to the departure end of the 4000 foot (1.2km) runway and prepared to takeoff. Now, I was taught to say, and teach my students to say, a standard matra before piling on the coals: “This is a standard/short-field/soft-field takeoff. If anything looks, feels or sounds unusual before rotation or I am not airborne by ABC, I will pull the power to idle and stop on the runway ahead.” This is to remove any requirement for decision making during the take-off roll, replacing it with a binary condition. I did this but obviously as a mantra rather than something to be thought about.

As I put the engine to full power the unusual noise, by now clearly mechanical, increased and, as I lifted off became alarming. At this point, with 3000 feet of runway still ahead of me, pulling the power to idle and landing straight ahead would have been easy. Instead I climbed.

It’s difficult to know what prompted this decision. Yes, I was running later than normal but had no deadline. Yes, pilots in general will climb to give themselves time to resolve a problem if something occurs in the air but not when they are on the ground—altitude is not a good thing if the tail’s about to fall off. Yes, I’ve flown thousands of hours and never had a mechanical problem….

Roger’s aircraft was just ahead of me and I called him and asked him to slow down, allow me to pull alongside and then inspect my aircraft. This he did and reported that he could see nothing on my right side. He then disappeared behind me and re-appeared on my left side where again he couldn’t see anything. I asked him to stay with me for the 30 minutes or so remaining to Rockcliffe to help if a problem occurred and he agreed. Francine, flying right-seat with Roger, then reported seeing something black streaming back from the pilot’s door and the puzzle was solved: not having any passengers, I had put my engine cover on the back seat and one of its straps, with a plastic buckle on the end, was dangling out and the slipstream was banging it onto the outside of the fuselage.

The buckle wasn’t a great problem, I opened the door and pulled the strap in and Roger had taken some great photographs (presumably as evidence for the coroner).

ptn.jpg

My decision making was a great problem. What would cause an experienced pilot in an aircraft he knows well under no time pressure to take off with an unidentified mechanical noise? The answer must be the “experienced pilot”, who had hundreds of hours of trouble-free flying, and the “aircraft he knows well”, which has been extremely reliable. I read many aviation incident reports and have taught myself, before doing things, to ask the question, “what would this look like in an incident report?” That didn’t work this time. If I read an incident report which said, “An experienced pilot took off in spite of a strange mechanical sound and lost control of the aircraft when one flap departed from the aircraft at 1000ft AGL” I would shake my head and ask how someone could be so stupid. But I did. And was lucky.

I keep a collection of decision problems for the classes I teach and it would be nice to think that this incident at LaChute would give me something to add to that collection. Unfortunately, the correct decision is so obvious that I can’t even use it there.

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Disclaimer

The author of this blog used to be an employee of Nortel. Even when he worked for Nortel the views expressed in the blog did not represent the views of Nortel. Now that he has left, the chances are even smaller that his views match those of Nortel.
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