Irish investigators probing a United Airlines Boeing 757-200 in-flight upset have advised the carrier to review its guidance on training for startle effects.

The inquiry into the 20 October 2013 incident, during descent into Dublin, occurred after the first officer mistakenly believed – as a result of a spurious low-airspeed reading – the aircraft was approaching a stall, and sharply pitched the jet nose-down.

Ireland’s Air Accident Investigation Unit says crews are “rarely” exposed to malfunctions and can be caught by surprise, and respond reflexively but incorrectly, if an unexpected event suddenly develops.

“One effect of the startle response can be the focusing of the individual on the causal stimulus to the exclusion of other events taking place in the cockpit such as warnings, alerts and communications,” says the inquiry. This can result in failure to recognise the nature of the situation and delay any recovery.

The lack of meaningful communication between the 757 pilots also meant that the captain was also vulnerable to a separate startle effect, which might have contributed to the failure to perceive an ‘airspeed disagree’ warning on the instrument panel.

This warning pointed to the true origin of the low-airspeed reading, a blockage of a pitot tube by ice crystals.

The inquiry notes that upset-recovery guidance underscores “proper and sufficient training” as the “best solution” for overcoming the startle factor, enabling pilots to supress the surprise and quickly shift towards analysis and resolution.

But while it has recommended that United reviews its material on upset recognition and startle – as well as underline to pilots the importance of cross-crew communication – the inquiry queries whether the ‘airspeed disagree’ alert provides sufficient warning to pilots.

The investigation lists several other incidents involving unreliable airspeed in which the crew did not recognise the ‘disagree’ alert.

Neither the Boeing 757 nor the 767 originally provided specific unreliable airspeed warnings, and the feature was only introduced in 2002. The warning triggers when a 5kt airspeed difference persists for more than 5s between the pilots’ individual airspeed indicators.

Pilots of the United 757, however, did not recall seeing the alert message or its associated master caution alarm. The inquiry is recommending, as a result, that Boeing looks into the effectiveness of the warning in attracting crews’ attention.

Source: Cirium Dashboard