Russian investigators are advising Boeing to consider amending autopilot logic to avoid the possibility of an aircraft’s automatically following a descent path incompatible with runway position.
The recommendation has emerged from the inquiry into the landing accident involving a MyCargo Airlines Boeing 747-400F at Bishkek on 16 January.
It resulted in 39 fatalities, most of them people on the ground, as the 747 automatically descended on a glidepath which overshot the runway, taking the aircraft past the airport and into a residential district.
Russia’s Interstate Aviation Committee found that the aircraft had intercepted a false glideslope, above the normal 3° descent path, because it had been too high on the approach.
Fifteen seconds after this false glideslope capture, the flight-mode annunciator registered a fault centred on the autopilot flight director system.
This indicated that the aircraft was no longer following the glideslope, and it would have generated warnings on the cockpit displays and the removal of certain flight-director information.
But the autopilot would not have disengaged, the inquiry says. Instead it would have maintained an inertial path, continuing to track a standard 3° descent path, regardless of the actual glideslope.
“The path will be maintained until a valid glideslope signal is regained or until the crew intervenes by disengaging the autopilot or initiating a go-around,” adds the inquiry.
Boeing equips its aircraft range with this inertial-path generation capability in order to allow the autopilot to continue an approach even if glideslope or localiser signals are disrupted.
“Without crew intervention the autopilot will maintain the inertial path until the ‘flare’ mode is engaged,” says the inquiry.
The crew of the 747 – which had been conducting the Bishkek approach in darkness and fog – did not intervene or attempt to execute a go-around until the aircraft had reached the decision height.
Even after realising the runway was not visible, the pilots allowed the aircraft to drift further down, and the autopilot’s flare mode engaged before the crew commanded go-around power.
The attempt to abort was too late to prevent a ground impact nearly 1km beyond the far end of the runway.
Investigators have recommended that airlines operating Boeing aircraft increase crews’ awareness of a possible switch to inertial mode by the autopilot during a glideslope descent.
Boeing should also “consider the practicability” of changing the autopilot’s logic to “prevent occurrences” of an inertial descent in situations when the approach path “does not allow landing in the appropriate area on the runway”.