French investigators are citing the deviation from its flight plan by a Proteus Airlines Raytheon Beech 1900D as the underlying cause of its fatal mid-air collision with a Cessna F177 RG light aircraft last year.
All 12 passengers and two crew were killed on the 1900D (F-GSJM) when it hit the single occupant Cessna (F-GAJE) over the Bay of Quiberon in north west France in July 1998 following an unplanned deviation by the airliner.
Operating as flight 706 from Lyon Satolas to Lorient in Brittany, the Proteus aircraft deviated from its filed flightplan to allow passengers to view the passenger liner, Norway, (previously France) moored in the Bay of Quiberon.
The impetus for the move appears to have come from a passenger on board the 1900D, who was standing in the cockpit doorway and directing the pilots down to the Norway, prompting the crew to abandon instrument flight rules (IFR).
As the aircraft was exiting its requested 360° turn around the ship, it hit the descending Cessna at an altitude of 2,000ft and virtually at right angles, banking slightly to the left.
In its report, the Bureau Enquetes d'Accidents (BEA) says: "This collision was mainly due to the absence of any visual detection by all crew of any other aircraft in non-controlled 'See and Avoid' G-class airspace.
"The fact that the Cessna 177 had its transponder switched off also meant that Lorient air traffic control (ATC) was unable to identify the aircraft on its screens or warn the Beechcraft of its close proximity".
Harsh criticism is made by the BEA of the 1900D captain's decision to abandon instrument flight rules (IFR) operation and adopt VFR procedures, even though this was approved by Lorient ATC.
"The Proteus Airlines company manual explicitly says that VFR operations are not allowed when transporting fare paying passengers and that the Captain must ensure the commercial flight plan is strictly adhered to," says the report.
"Implicit in company guidelines is the fact that during critical phases of the journey, crew members must only be preoccupied with what is happening during that flight and not be distracted by any other incident, no matter what it is".
The BEA strongly recommends that the DGAC make IFR operations mandatory for revenue passenger flights, unless it is found absolutely necessary to abandon them and that safety is "systematically prioritised" by pilots even if commercial or profit motives are otherwise apparent.
Key to avoiding any such future fatal accidents, adds the BEA, is the immediate implementation by the French CAA (DGAC) of dedicated IFR air space and the installation of traffic alert and collision avoidance system (TCAS) equipment on all aircraft involved with transporting the public.
The latter measure must take place as soon as possible, says to the accidents bureau, although it is due to come into effect in Europe anyway during next year. As it happens, the Cessna 177 was operating with its transponder switched off, so that the 1900D's BF Goodrich-supplied TCAS 1 system (which provides only traffic information and not resolution advice) would not have detected the smaller aircraft in any case.
Transponders are recommended by the BEA for inclusion on all aircraft. It made a similar recommendation following February's incident when an Airbus A320 collided with a non-transponder equipped glider near Montpellier.
The Cessna 177 had such a transponder, but its 70-year old pilot elected not to use the equipment. The BEA comments: "The Aeronautical Manual states that any aircraft with an altitude reporting transponder, must, in the absence of any ATC instruction, have the transponder set to Code 7000 and activate the 'altitude report' function at all times".
Accordingly, the BEA recommends that all general aviation pilots familiarise themselves with the Aeronautical Manual's VFR and transponder instructions, as well as ensuring that such transponders are always active, unless security reasons dictate otherwise.
Despite permission being granted by Lorient ATC for the Proteus airliner to leave its plotted course and descend from FL50 to 3,700ft, both pilots appear to have been distracted by the standing passenger behind them as well as by the liner itself in the bay.
Their subsequent decision to descend to 2,000ft under VFR without clearance exacerbated the danger.
Furthermore, the 1900D was in contact with Lorient approach control, while the light aircraft was receiving airport flight information system (AFIS) advice from nearby Quiberon control - meaning the crews had no opportunity to be aware of each others' presence by radio monitoring. The Cessna pilot also died in the incident.