Remember the first Boeing 777 crash ever? I wrote about it here when it happened on the 17th of January. I initially suspected that it was wind shear that caused the crash. But it wasn’t. I recently read an interesting article and the author blamed it on pilot error. Here’s what he thinks happened.
Flight BA038 was on a holding pattern above Heathrow when the ATC told them that they could come in for a landing early. In order to make that landing, they would have to slow the plane down considerably. There are a few ways of slowing a plane down. Speed brakes (air brakes) and flaps can be used. Engine power can be reduced also, but only to a certain extent and under certain conditions only.
The pilots made a big mistake when they reduced the engine power below the recommended threshold for safe flight. You see, aircraft turbine engines are very different from car piston engines. A car’s engine can rev from idle to max rpm in a matter of seconds. The Boeing 777 engines (Rolls Royce Trent 800) on the other hand requires up to 12 seconds to spool up from idle and give maximum thrust. The pitch up attitude (nose up) of the aircraft during approach made matters worse because the airflow entering the engine inlet is at an angle and airflow to the compressors is affected. This coupled together with the low engine rpm creates a very favorable condition for stalling and surging of the engine to occur. Stalling and surging can cause a loss of thrust and can also cause an engine to flame out and shut down. In more serious cases, the whole engine can be destroyed.
So back to our situation. The pilots of flight BA038 pulled back on the throttles too much to descent quickly. Doing so caused the engines to spool down and reduce the output thrust. The plane slowed down and in order to maintain lift, they pulled back on the control columns, causing the plane to fly at a nose up attitude. This positioned the engine intakes at an angle to the incoming air which as explained above affects the airflow into the engine.
During the approach, they lowered the landing gears and flaps and with the extra drag caused by the landing gears and flaps, the plane started to drop. They realized that they needed more lift (read: more thrust) to make it to the runway. Remember at this point the engines are at low rpm and the engine intakes are not at the proper angle with the incoming airflow. They pushed the throttles forward to increase thrust, but as the engine tries to spool up, the unfavorable conditions caused them to surge. When a surge happens, there is a loss of thrust and loud booms can be heard. With a loss of thrust, there will be a loss of lift. Passengers recalled hearing loud sounds coming from the engines and also an unusually steep descend (quote “we just dropped”). The captain claimed that “there wasn’t power” when he needed it.
If you’ve been following what I’m trying to say, you’ll know why there wasn’t any power when the throttles were opened. The engine needed time to recover from the surge, and at such low rpm, it would have taken 12 seconds for it to go to full thrust. 12 seconds was time that they didn’t have. To make the story short, the plane lost lift, descended too quickly and hit the ground before it reached the runway and became the first Boeing 777 to crash.
As the author mentioned, they shouldn’t have reduced thrust too much during approach and should have trusted the computer when it insisted that thrust should be maintained at a higher level. In fact, I had a chat with virtualmystic and he said that normally throttles shouldn’t be touched during the approach. Any deviation of the aircraft from the glideslope (ILS) should be corrected by using the control column as it gives better response.
Disclaimer: I don’t guarantee or claim that the above are 100% correct because I’ve not read the official report or seen the FDR data. It’s just a speculation that I find interesting and logical. But if you think I’m wrong, do drop me a line. I’ll be happy to hear your reasoning. As always, happy flying! Continue reading