This little boo boo occurred at KATL this morning. Fortunately there were no passengers involved and no injuries reported.
According to the “News” two mechanics were on board to test the engines and there was some problem with the brakes. Some mechanics are qualified to taxi the airplanes and are authorized to do so for the purpose of re-positioning the aircraft or for engine run ups. For obvious reasons, the engines cannot be advanced to takeoff thrust at the gate. Equipment and human beings would be blown all over the terminal area. I have no comment on this particular incident because I have no clue what happened and to speculate would be ridiculous and disrespectful to those involved. It did cause me to think about some of the experiences I’ve had with the braking system however and I thought I’d share some basic info with you.
The brake system on an airliner can be operated from several different hydraulic sources and can be pressurized from engine driven hydraulic pumps as well as an electrical auxiliary pump. There are also accumulators built into the system that can store enough pressure for several brake applications in case all else fails. Sounds foolproof doesn’t it, but accidents can still happen.
One way is a malfunction of the anti-skid system. If a main gear wheel locks up, the system removes pressure in order to prevent the tire from blowing out. Anti-skid circuits are normally automatically de-activated below about twenty knots and of course warning lights are available to announce malfunctions.
Here’s something else to think about. At takeoff power, the airplane will sometimes move even if the parking brake is set and the wheels don’t turn. If the pavement is wet or covered with ice or snow, you can almost count on it. If only one engine is run up, the airplane will also turn as it moves due to asymmetrical thrust. The classic example, of course, is the accident involving a 747 in Alaska many years ago. The airplane was cleared into position and hold on an icy runway and then the parking brake was set. When cleared for takeoff, the power was advanced and the airplane accelerated down the runway with the parking brake still applied. The wheels never turned and the airplane never reached takeoff speed.
Even a more normal situation can be complicated sometimes. I’ll use the MD-11 as an example. During ground operations in icing conditions with engine anti-ice on, the engines have to be run up to 50% at least every 15 minutes in order to shed accumulated ice from the spinner and fan blades. When this is accomplished on an iced over taxiway, it can be a problem. First of all, if you run all three engines up at once, you might move forward and the airplane in front of you might become an obstacle. The airplane behind you will probably have words for you too, when he is being bounced around in the hurricane you create while being bombarded with snow and ice. The trick, of course, is to turn the airplane at a forty-five degree angle on the taxiway before running the engines up. You try to run them up one at a time and hope the airplane doesn’t slide sideways on the ice. If that’s a problem and you have to run the wing engines up together, the narrow taxiway leaves little room for error if the airplane moves forward.
Sometimes you just can’t win and boo boo’s happen.