June 29, 1994
Boeing mystified over 'violent' 747 stall
Boeing has failed to find the cause of an incident in which a Singapore Airlines (SIA) Boeing 747-400 stalled while at cruising altitude over Bucharest on a Singapore-Heathrow flight (Flight International, 3-9 November, 1993).
Independent reports from two private pilots, who were passengers, subsequently stated that the incident was far more violent than Flight International's report or the SIA statement at the time indicated.
The incident involved cockpit audible stall-warnings which could be heard in the upper-deck cabin, then buffeting, followed by two separate stalls during which the aircraft descended rapidly, losing "several thousand feet" in controlled airspace. SIA would admit only to an unexplained loss of airspeed followed by a pilot decision to descend to increase speed.
One of the pilot passengers reported that some flight attendants were thrown off their feet and that drinks hit the cabin ceiling.
An SIA source says that the pilot and co-pilot were puzzled by an apparent disparity in the airspeed readings of their respective instruments - one of which appeared to be receiving "frozen" speed data. The incident occurred as they tried to decide which instrument was giving correct readings.
Boeing says: "Working with the customer airline and equipment manufacturers, we performed tests on aircraft equipment which we thought might have contributed to this anomaly. We examined the air- data computer, pitot-static probe systems and other equipment. Nothing was discovered that would explain the anomaly.
"The equipment was removed and replaced, pitot-static lines drained and flushed, and the aircraft returned to service. There has been no recurrence." Boeing adds: "We're still working with the airline to see whether we can determine the cause. We have not been informed of other instances like this."
from: Flight International
(apologies if this is off-topic, but it does cite a possible pitot issue)