The NTSB is recommending modifications of Honeywell flight management system (FMS) software that would provide warnings to pilots if they try to enter inconsistent weight and performance information.

In a letter to FAA Administrator Marion Blakey, the NTSB said the recommendation was based on two serious tailstrike incidents in which pilots entered incorrect information into FMS computers. One incident, on March 12, 2003, involved a fully loaded Singapore Airlines Boeing 747 at Auckland, New Zealand, that rotated on takeoff some 33 knots slower than the required Vr of 163 knots. The NTSB said the calculation error was the fault of the copilot, who inadvertently entered a fuel weight figure that was 220,000 pounds less than the airplane’s actual fuel weight. The Safety Board also faulted the Honeywell FMS, however, for allowing the incorrect figure to be entered and calculated in the first place.

Another incident, in the fall of 1998, involved a Delta Air Lines MD-11 that experienced a tailstrike on landing at Portland International Airport in Oregon. Again the NTSB cited pilot error, but also faulted the FMS for allowing input of a weight figure that was below zero fuel weight.

The NTSB asked the FAA to require software modifications that would warn pilots of unsafe takeoff reference speeds and prevent entry of airplane weights resulting in landing weights below zero fuel weight.

Honeywell stood by its avionics equipment, but said it would look into the matter further. “Our flight management systems have proved to be safe when operated according to the flight manual,” a statement read. “However, we will work with the aircraft manufacturers and government regulators to look into ways of reducing the probability of human error, and we have the technical capability to meet the safety goals proposed by the NTSB.”