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Warning on cabin alert ‘confusion’

Similarity of alarm sounds highlighted as Greek accident probe focuses on failure of Helios aircraft to pressurise

Boeing has acted to reduce the likelihood of flightcrew misinterpreting a cabin altitude warning, following a request by the investigator leading the probe into the Helios Airways Boeing 737-300 crash in Greece on 14 August.

Although the investigation has a long way to run, evidence gathered so far suggests the basic cause was the aircraft’s failure to pressurise, resulting in the flightcrew losing consciousness.

The Greek Air Accident Investigation and Aviation Safety Board (AAIASB) has known for weeks that the cabin altitude (CA) warning horn sounded when the aircraft was passing through 14,000ft (4,270m) (Flight International, 23-29 August) and that it was not cancelled for the rest of the flight from Larnaca, Cyprus to Greece. But the investigators did not know why the crew did not cancel the strident warning sound.

Now it seems the AAIASB is concerned the pilots may have been confused by the fact that the automatic aural warning for an incorrect take-off configuration and the aural alert triggered when cabin altitude rises above 10,000ft make the same sound.

Boeing says its reminder to crews about the differences in the warnings “doesn’t mean [this] is necessarily the cause [of the Helios accident]”, and emphasises that “the investigation is still ongoing”. But the fact Boeing issued its “multi-operator message” to 737 users at the AAIASB’s request is widely seen in the industry as a tacit acknowledgement that cabin pressurisation and the CA warning are the focus of the investigation.

Because the alert sound is identical for both the CA and take-off warnings, the Boeing message reminds crews that the take-off configuration warning horn can sound only when the aircraft’s weight is still on its wheels. If the same alert sounds in flight, it is the CA warning.

The CA alert system is designed to warn the crew if cabin altitude has not been set at 8,000ft. The alert is activated at 10,000ft in case the system fails or the crew have, for example, turned off the pressurisation before take-off to gain more engine power, but then forgotten to reset it.

The effect of failure to pressurise is that anyone in the aircraft who does not don an oxygen mask will gradually lose consciousness. In the case of the Helios flight, the aircraft continued to fly on autopilot, the profile programmed into its flight management computer, taking it at a cruising level of 34,000ft to its final approach fix south of Athens airport where it entered the holding pattern.

That profile has been confirmed by the AAIASB, and Lockheed Martin F-16 fighter pilots sent to investigate the 737 reported seeing passenger oxygen masks deployed, but the copilot was slumped in his seat. The captain was not visible.

Full details of what triggered the eventual descent to the crash site are not fully understood, but AAIASB chief investigator Akrivos Tsolakis says the engines stopped from lack of fuel as the aircraft descended through 7,000ft.

All six crew and 115 passengers were killed at impact 30km north-east of Athens.


Setting of pressurisation control panel in spotlight

A report submitted to Cyprus police by Helios’ British chief engineer Alan Irwin about pre-flight maintenance checks on the Boeing 737 that crashed later that day has revealed a possible clue as to why the aircraft failed to pressurise.

Because the cabin crew had warned on a previous flight that the rear service door was “noisy”, the engineers carried out an on-ground pressurisation of the cabin before its departure on 14 August to see if the door was leaking, says Irwin’s report. This would require the use, in manual mode, of the same pressurisation control panel the crew would use. Having carried out the check successfully, the engineers opened the pressure relief valves to depressurise the aircraft.

The AAIASB has not reported whether the pressurisation selector switch was found in manual or automatic. Normal flightcrew pre-take-off checks would see them select cabin altitude to 8,000ft and the pressurisation selector switch to automatic.

Irwin’s report describes a subsequent radio call from the flightcrew when they were airborne to Helios’ engineering department on the company frequency. Just after the CA warning had activated, another alert sounded, this time warning that the avionics bay cooling fans were not operating.

Helios’ engineering department said the captain’s request was not clear, and they asked him whether the pressurisation panel had been reset to automatic from manual. His response was to ask where he could find the circuit-breaker for the avionics bay fans. Engineering told him it was behind his seat.

The aircraft was still climbing, and that was the last communication the captain made.

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