Apparently, for the second time in a matter of weeks, a crew let an autopilot stall an aircraft causing a fatal accident. In my last article I stated that the aircraft stalled and that this week we would learn how/why it stalled. I was shocked to realize it was an autopilot induced stall, again. Automation can NEVER replace a well trained and rested crew.
How did it happen? Again, we will have to start with some basic systems knowledge. A radar altimeter is a precise instrument that measures an aircrafts actual height above the ground (AGL), by transmitting a beam to the ground and measuring how long it takes to bounce back to the receiver. The time measurement is then converted to feet on the cockpit presentation. The barometric altimeter measures the height of an aircraft above sea level (MSL). They are only the same when landing at an airport at sea level. For example: in Denver (5,000 foot elevation) if the barometric altimeter is indicating 5,200 feet, then the radar altimeter will indicate 200 feet. I go into detail on the different airspeeds and altitudes in my novel so that the reader can get an appreciation of why they are important.
Auto throttles maintain the speed set, in normal flight conditions. The system is like cruise control in a car.
Auto land (coupled approach) is a mode of the auto pilot that flies the approach and lands the aircraft. It integrates with the auto throttles and ILS (Instrument Landing System) to fly a constant airspeed, glide slope and course all the way to the runway (the systems talk to each other). At 50 feet above the runway AGL (measured by the radar altimeter, Captains side) the auto throttles will go into landing flare mode and go to idle. The auto pilot will command the nose to rise and maintain the glide slope and course provided by the ILS until weight on wheels (touchdown).
The accident: All engines and flight controls were operating normally and responding to crew inputs. There was adequate fuel onboard. The Captains radar altimeter was giving erroneous indications (-7 feet), noted by crew. Auto pilot b (copilots) and auto throttles were engaged for a coupled ILS approach. The right radar altimeter (copilot side) was functional.
At 2,000 feet the auto throttles processed the bad information from the left radar altimeter and commanded the engines to idle. THE THROTTLES REMAINED AT IDLE FOR 100 SECONDS (1 minute and 40 seconds). Over that period of time the aircraft decelerated 40 knots (25-33% of total indicated airspeed). Inexplicably, none of the three pilots in the cockpit reacted. At 500 feet AGL the aircraft went into stick shaker (stall warning system). The crew selected full power, however due to spool up time of the engines and low altitude they were unable to recover.
It is widely being reported the altimeter caused the accident. It caused the throttles to go to idle, the accident occurred over two minutes later. A perfectly good airplane with a very minor failure.