Monday, October 13, 2008
Skip foward 60 seconds for an example of just how wild a rejected takeoff can be.
A rejected takeoff is one of the most dangerous situations you might encounter in aviation - it's not easy to slow a large heavy object from over a hundred mph to zero in a few thousand feet. We use thrust reversers which help a little, and we use speedbrakes to increase our drag and slow down faster, but the job mostly falls upon the aircraft brakes, which are usually made with all sorts of high-tech expensive materials like carbon fiber ceramic and Kevlar (same stuff they use in bullet-proof vests), but can still be completely demolished in a single rejected takeoff. Brake temperatures can reach 1,800 degrees Farenheit due to friction, which which is on the high end of the temperature found in a cremation oven at a funeral home (thanks to Google for that morbid little search result).
Anyway, when we push the throttles forward in our jet, we have already computed and marked several airspeeds on our airspeed indicators. One of them is V1, which is also known as "Decision speed". On the take-off roll, once we accelerate past V1, even if an engine blows it's safer to continue the takeoff than it is to stomp on the brakes and attempt to stop on the remaining runway. If our plane is heavy or if it's hot outside then it will take more runway to come to a complete stop. Before we attempt any takeoff we have done the math (actually a computer program does the math, but you get the point) and made sure that the runway we will be using is longer than the runway we actually need. If we are doing a charter flight, we need a 60% distance buffer on top of the minimum distance calculated by the computer program.
There are some things the calculations can't take into account, the main one being if the pilot decides to reject the takeoff at a speed above V1. If that happens, then there is no data out there that tells us how much runway it will take to slow down, or if we will slow down at all - at a certain speed our kinetic energy is such that the brakes will fail before they absorb enough forward motion to keep us from going off the end of the runway. In our training, we get it pounded into our heads that after we reach the V1 speed on takeoff, we are going flying unless a wing comes off. But I have heard more than a few accounts from pilots of them rejecting takeoffs well above V1 for various reasons, and most of the time the runway is still long enough. Most of the time.
Now, this accident happened a couple of weeks ago in Cabinda, Angola. We don't know the details, but from an accident-investigation or "armchair quarterback" point of view, I can see several things that *might* have resulted in the calculated accelerate-stop distance appearing less than the distance it actually took them. How many factors do you see at work here?