December 2011
incidents
- Responsibility
- 1 December 2011
- By Dr Guy Gratton
- The Boeing C-17 Globemaster III, at US$185,000,000 each is one of the most expensive aeroplanes ever built, and with a 585,000lb maximum take-off weight, it is one of the largest. Given all that, it’s hardly surprising that the aeroplane is a crowd-puller at military airshows across the world.
Early on the evening of 28 July 2010, the crew of a C-17 registration 00-0173 and callsign Sitka 43 got airborne from Elemendorf-Richardson Air Base in Alaska to practice for an airshow a few days later. The captain was a highly experienced and respected C-17 pilot with 3,251 flying hours, 974 of those as an instructor, 866 on type, and 17 hours in the last month. He’d very recently trained as a display pilot. Around him was a highly experienced crew with an average of over 3,000 hours each – and all of them were fit, current and healthy. So we have exactly the sort of team of professionals you’d hope to be in charge of one of the most complex pieces of machinery ever built.
The aircraft was expected to get airborne straight into a climb at minimum permitted climb speed to 1,500ft, then a display routine which included turns up to 45 degrees of bank.
What actually happened was that the aircraft was climbed to 850ft at 40 degrees nose-up, which gave a maximum speed of 107 knots and put the C-17 26 or more knots below the minimum climb speed of 133 knots, before banking 60 degrees left to change heading by 80 degrees, levelled for a few seconds, then turned right reaching 60 degrees of bank before the stall warners (a voice alert as well as a stick shaker) went off in the cockpit. Despite the warner, the pilot kept pulling the aeroplane into the stall and increased right rudder as well as pulling the stick fully back. The aeroplane stalled – rolled into the turn to beyond 80 degrees, and started to descend at 9,000 feet per minute. That gave little time before the inevitable – the aeroplane descended into forest a short distance from the runway, killing all on board, and destroying the aeroplane. In trying to understand this, lets start with some flight mechanics. For any given configuration, any fixed wing aeroplane will always stall at the same angle of attack. The stall is a condition where the aeroplane ceases to be fully controllable in all three axes, and is usually accompanied by a high rate of descent. A stall close to the ground in any fixed wing aeroplane is a very high risk event – and causes loss of many aeroplanes and crew annually. Any modern large aeroplane, and even most small aeroplanes, will provide an artificial stall warner which triggers a small margin below the critical angle of attack. In the C-17 this is a particularly capable system which allows for configuration, engine power, sideslip, airspeed and altitude – and several more parameters besides.
As an aeroplane banks, neither the stalling angle of attack nor the stall warning angle of attack change. But the speed at which the aeroplane reaches those goes up with the bank angle – and not in a straight line. If you work it out you’ll find that whilst at the planned 45 degrees bank angle the stall speed would only have gone up by a fairly small 18%: another 15 degrees added to the bank angle would have increased the stalling speed by another 23%, and another 10 degrees after that brings the stall speed 70% more than the original at 70 degrees of bank. So bank angle control at low speeds – and this aeroplane was at low speeds – is really important. However, this pilot and his crew were not inexperienced and they knew these facts extremely well. So, how did they get to this position?
It seems that this comes down to a combination of a pilot who was far too keen to put on a good show – and was allowed to try by relaxed supervision that assumed he could be trusted – the USAF official report says: “Because he was an accomplished aviator, leadership allowed him to operate independently with little or no oversight.” – this is despite what came out in the investigation as many incidences of his busting height, bank angle and stall warning limits to get a better display or show other pilots how to get the most from their aeroplane.
In the flight itself, it also became clear that the co-pilot and another pilot in a jumpseat as safety observer could see that this was getting out of hand. Within a few seconds of the stall occurring the co-pilot had said “not so tight brother”, whilst the safety observer said three times “watch your bank”. How did this experienced Captain, who I’m sure both learned and taught teamwork and monitoring issues regularly – and knew very well the implications of exceeding limitations, ignore this feedback, and audible stall warner, and the stick shaker – and keep pulling to the death of himself and three colleagues, as well as the loss of a valuable aeroplane? The man was neither a beginner nor a fool, but he did it.
Going through the report, I find the phrase “pilot error” repeated several times – it’s one that military organisations love using, because the military system, nor the military equipment couldn’t possibly be at fault. Could it?
Well maybe it could – this pilot had been eating into the safety margins provided by operating data and warning systems for a long time. I can’t believe that this was the first time this pilot had bust limits, nor the first time that another experienced pilot had warned him about this. And clearly, every time up to 6:23pm on July 28th 2011, he’d got away with it, and his fellow pilots had dismissed it as the acceptable exuberances of an experienced pilot who knew what he was doing. Well they weren’t, and he didn’t – very clearly. He was the show-off who got away with it one time less than he needed to. Ultimately he was the captain, and it was his responsibility to keep that aeroplane safe; a duty in which he failed. However, it was the duty of every instructor, every co-pilot, and every display supervisor he’d worked with to help this man stay safe, and stay within the rules, professional knowledge and safety margins that were there to prevent an accident just like this – and in that they failed just as much as he did.
