United States Standard Atmosphere Table, pressure ratios were determined and applied to the existing QNH[1] for the altitudes 1,240 and 916 feet. The pressure required at the Pitot head to generate an indicated airspeed of 130 knots at 1,240 feet was also calculated.
It was then assumed that the aircraft descended to 916 feet and the static system continued to sense a static pressure equivalent to 1,240 feet, and the pilot controlled his aircraft so as to maintain an indicated airspeed of 130 knots. This would require a constant Pitot system pressure. With these conditions, it was found that at 916 feed, when the indicated airspeed was maintained at 130 knots, the actual airspeed would have been 100 knots.[2]
Inasmuch as an actual speed of 100 knots is very close to the stalling speed of the aircraft in the landing configuration, it is highly unlikely that such a condition would escape the notice of the pilots. It is therefore significant that no mention of any such problem was made during the approach. The accuracy of the airspeed instruments is further verified by the time taken to fly from the outer marker to the point of impact. In view of the above, the Board concludes that both the indicated and actual airspeeds were in the area of 130 knots during the approach.
The only explanation which would reconcile an inaccurate barometric altimeter with an accurate airspeed indicator is that there was an error in the Pitot system which roughly offset the error in the static system. The Board, however, is not aware of any phenomenon, atmospheric or otherwise, which could produce such an offsetting error, nor was there any evidence thereof uncovered during this investigation. In this connection, it should be noted that long-term research is underway to determine whether flight and weather conditions can lead to misinformation from instruments connected
- ↑ QNH is the altitude above sea level based on station barometric pressure.
- ↑ The above calculations are set forth in detail in Appendix E. As further calculated in that appendix, a static system error which would cause a 200-foot altimeter error would produce a corresponding airspeed error of -17.5 knots (i.e. when the airspeed indicator read 130 knots, the actual airspeed would be 112.5 knots).
[Footnote continued] approach at 130 knots, and the first officer's callouts were within ±7 knots of that figure. The approximate value was also recorded by the airspeed trace on the flight data recorder.
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