On 20 December 2009, a McDonnell Douglas MD-83 (F-GMLU) being operated by Blue Line on a positioning flight from Paris CDG to Kuwait with six crew on board requested a climb to FL 370 which was approved. Soon after levelling in day Visual Meteorological Conditions (VMC), aerodynamic buffeting began and, having realised that the speed was low, the crew initiated a descent to FL 350 during which the buffeting ceased. The remainder of the flight was uneventful.
An Investigation was carried out by the French Bureau d'Enquêtes et d'Analyses (BEA). Flight Data Recorder (FDR) data was successfully recovered to assist the Investigation.
It was established that during pre-flight preparations, the First Officer, designated as PF, had mistakenly entered the aircraft ZFW of 86,520 lbs into the Performance Management System PMS) in place of the GWT (Gross Weight) of 129,673 lbs. Since the PMS requires that the GWT value should be between 88,000lbs and 170,000lbs, the entry led to the annunciation of the CHECK GWT message. The First Officer stated that “he then re-entered the weight value and the error message disappeared” but that he did not remember what value was entered. It was confirmed that the aircraft commander had not cross-checked the weights entered.
It was noted that prior to departure, the FPL had shown a cruise level of FL 350 based on a GWT which was 5000 lbs below the actual GWT and considered that “the crew might have been alerted when the PMS proposed a cruising level higher than FL 350 (FL 370) even though the aircraft was heavier”.
It was also noted that the crew had stated afterwards “that the poor meteorological conditions on departure (snow on the ground) and problems with the airline’s preparations had resulted in a significant delay and thus a high workload”.
Once airborne, a climb to FL 260 was made and, upon reaching, the crew requested and were approved for continued climb to FL 370. Shortly afterwards, whilst passing FL 300 at M 0.77, ATC asked if it would be possible to reach FL 370 within 4 minutes. Having consulted the PMS, which based on the erroneous data input indicated a minimum speed of M 0.59, an affirmative reply was given and the PF selected a speed of M 0.65. When the 4 minute target was evidently not going to be met, the crew and that of the other flights involved in a potential conflict were both asked to turn 20° left. As the aircraft came out of the turn, the angle of attack was 3° and increasing as the AP maintained M. 0.65. The aircraft levelled at FL 370 but just over a minute later, the A/T mode changed to ‘MACH ATL’ indicating that the thrust required to maintain M 0.65 was greater than the maximum thrust available. The crew reported that they had felt buffeting and had “thought that the aircraft was behind the power/speed curve”. The AP was disconnected and a descent was commenced with the buffeting ceasing as the speed increased. The aircraft was levelled at FL350 and the flight was completed without further event.
The primary concern of the crew - and the reason for initiating a descent - appeared to have been the recognition of low speed rather than the recognition of proximity to a stall. Recorded data confirmed that on reaching FL 370, the aircraft had been close to stalling with the speed M 0.64 and the angle of attack 5.2°. It was noted that the airworthiness type certification requirements applicable to the MD83 under the 1977 version of FAR 25 prevailing at the time required a 7% margin in speed between the stall and the warning of a stall, the latter to be provided either by aerodynamic buffeting or by a stick shaker or equivalent system. It was further noted that although MD-80 series aircraft are fitted with a stick shaker to warn of an imminent stall at low altitude, at high altitude, the warning is aerodynamic buffeting.
The performance of the aircraft at its actual weight at the time of the incident rather than the one the crew had assumed based on the erroneous data entry was reviewed by the Investigation. It was found that at the actual weight, the onset of buffet had been as expected and that the speed and stall level information provided by the PMS had been consistent with the weight input during the pre flight preparation.
It was considered that the stall level of FL370 and the relatively low speeds generated as feasible by the PMS had not alerted the crew because they had expected that, at the light weight expected due to the fact that it was an out of service positioning flight, there would be good performance relative to a fully loaded aircraft.
The occurrence of a very similar event four years earlier to an MD-82: MD82, en-route, near Machiques Venezuela, 2005 which became a fatal accident when the aircraft stalled and the crew could not recover from it was noted as were the relevance of safety recommendations arising from that Investigation in respect of improving crew awareness of aircraft performance ceiling and high level aerodynamic stall buffet.
The formally-recorded Conclusion of the Investigation was that “the incident, involving a near stall, was brought about by an error inputting the weight into the Performance Management System (PMS)”.
Safety Action by the aircraft operator was reported to have been taken as follows:
- verification of pilot knowledge of the magnitude of operational values
- introduction into CRM training of a module specific to charter flights
- introduction into CRM training of the importance of briefings and cross-checks.
- issue of a reminder of the requirement to complete a new performance card if the actual take off weight is more than 3 tonnes greater than the provisional take off weight entered during flight preparation.
- improvements in the procedures for cross checks.
The Final Report of the Investigation was issued in a French language version only but is also available here on SKYbrary in an unofficial English language translation. No Safety Recommendations were made.