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?


nec Timide said...

Do turbo props have to compute accelerate stop distances? I thought that was a jet only thing, though obviously it is something any multi-engine operation can benefit from.

Blake said...

Nec Timide,

I had to calculate accelerate/stop distanced on my standard piston multi engined Piper Seminole.

In fact, I had to demonstrate how to do it as part of getting my mutli rating.

Here are a few things I can think of that could have contributed to a mis calculation:

1. Temperature. The temperature in the ATIS is given from a thermometer that is located a certain height above the ground somewhere on the field. The *actual* temperature above the runway can be 10s of degrees warmer. This can skew your calculations.

2. Weight.. Maybe they mis calculated how much weight was on board.

3. How long before they feathered/reversed the props? If at all? Immediately, or a few seconds after reducing power and stomping on the brakes.

4. Runway length. Maybe the runway was shorter than in publications due to construction? Pilots did take into consideration the amended length of runway that was in a NOTAM.

Ryan said...

Jeez! I kept saying to myself, "shut it down", when he went into the dirt. That's as real as it gets there.

zb said...

The sand was a great helper in terms of braking and keeping the brakes from setting anyting else on fire.

chris said...

Taking off downwind can't help.

Dagny said...


That's some good piloting there Lou.

Anonymous said...

Jeez, how did I know this was going to be a Russian plane?