Finnish investigators have determined that a Gulfstream G150 captain was killed when explosive depressurisation blasted open the aircraft’s door as he attempted to enter the jet at Kittila.

The Private Airlines Germany aircraft (OE-GKA) had been parked outside the previous evening, and its outflow valve was left closed – possibly by using the ‘ditch’ button – in order to prevent ingestion of blowing snow.

This informal practice of using the ‘ditch’ button to close the valve was “not authorised”, says the Finnish Safety Investigation Authority, because the button is intended only for emergency use. The captain did not inform the first officer of the valve’s closure.

When the crew – comprising the captain, first officer and cabin assistant – returned to the jet the following day, for a ferry flight to Ekaterinburg, the captain entered the cockpit and started the auxiliary power unit, generating electricity and bleed air for heating.

The first officer, meanwhile, was outside brushing snow from the aircraft and, shortly afterwards, was joined by the captain, who briefly re-entered the jet to fetch gloves. When he exited the aircraft, the captain closed the door, leaving the cabin assistant inside.

Investigators state that the assistant started feeling a sensation of pressure in her ears and chest, and sought to attract the pilots’ attention by knocking on the window.

As the captain attempted to open the airstair-type door, by pulling on the activation handle, it suddenly exploded outwards, inflicting fatal injuries on the captain and knocking the first officer to the ground.

While the cabin assistant testified that she had fainted at the moment of the accident, the inquiry believes she actually lost consciousness after colliding with the cockpit’s aft bulkhead.

The inquiry believes that the captain, when starting the APU, pressed the ‘ditch’ button with the intention of opening the outflow valve.

But it states that deselecting ‘ditch’ would not have changed the outflow valve position, because automatic pressurisation had been selected at the time.

“It may well be that the captain…assumed that the outflow valve would open with pressurisation in automatic,” says the inquiry. Had the pressurisation system been in manual mode, the outflow valve would have opened when the ‘ditch’ button was pushed.

Investigators state that the operator complies with Gulfstream’s cold-weather operations manual. The temperature in Kittila at the time of the accident, on 4 January this year, was minus 22°C.

According to the manual the aircraft is to be heated by using the APU’s bleed air, and the heating process can be quickened by closing the door. There is a supplementary checklist which warns that the outflow valve should be fully open before the door is closed, if the APU is running, but the inquiry has recommended updating the manual with a specific caution in the section on closing the door during heating.