UK regional operator Flybe has introduced new procedures to check Bombardier Q400 engine panels are properly closed after two incidents involving the same aircraft in five weeks.
The turboprop (G-PRPC) suffered puncture damage to its vertical fin when an access panel on its left-hand engine was torn off, as the Q400 took off from Manchester on 14 December last year.
As the panel struck the aircraft it also damaged antennas and inspectors found impact marks on the leading edge de-icing boot.
The large panel, located just aft of the propeller, is hinged at the top edge and has sets of latches in the centre and on the lower edge.
In a bulletin the UK Air Accidents Investigation Branch states that the panel was "not securely closed" by an engineer following overnight maintenance work.
It says the failure to notice the incorrect latching might have been a consequence of the engineer's technique of closing the upper latches first, combined with his elevated viewpoint and the dark colouring of the nacelle which together disguised a shadow cast by the still-open panel.
The operating crew similarly did not detect the problem during the pre-flight inspection, which was conducted in darkness.
Investigators found that the same Q400 had lost an access panel from the same engine just five weeks earlier, on 9 November, during a flight from Belfast City. Damage was discovered to the wing de-icing boot and wing skin panels.
In the aftermath of the Belfast incident Flybe issued a notice to engineers requiring an independent check of panel security after the completion of work. But the maintenance before the Manchester flight in December was performed by a third-party provider, which stated that it had been unaware of this notice.
Flybe subsequently revised the notice to introduce a new procedure involving placing a sticker on the lower edge of the panel after closure, to provide "visual and tactile confirmation" to the engineer that the panel is secure. It is also requiring that subcontracted maintenance organisations receive copies of notices to engineers.
Bombardier told the probe that nine other instances of in-flight panel loss had been recorded by Q400 operators over the course of December 2005 to January 2017.