Recommendation Date
Recipient Name
Boeing
Text
Implement a FMS software change on all various Boeing aircraft models that ensures any entries (such as V speeds and gross weight) that are mismatched by a small percentage are either challenged or prevented.
Reply Text
According to the NZ TAIC report, the load sheet provided to the crew contained the correct weights for the flight and the correct weight was entered into the FMS. However, the crew used an incorrect weight to manually calculate the takeoff speeds (V speeds) from airport analysis charts. The incorrect V speeds were entered into the FMS and used by the crew during takeoff, resulting in the tail strike.
Background
This event is another example of incorrect takeoff speeds, which has previously been identified by Boeing as an issue for the industry. The common feature among these cases is that the takeoff speeds used by the crew are inappropriate to the specific operating conditions (actual weight, runway length, etc). The error or errors leading to the incorrect speed can happen at various points along the computational path, which consists of both manual and automated operations. In all cases, the results are the same - a takeoff is attempted with rotation at an inappropriate speed. The consequential risks to the airplane (tail strike, overweight takeoff, increased runway length, reduced manoeuvre margin to stall, reduced climb gradient, etc) are the same regardless of the specific error that led to the incorrect V speeds. Boeing is working to ensure that adequate and appropriate defences are in place to reduce the possibility that such errors are made or propagated.
Prior to the Auckland event, Boeing had reviewed the takeoff speed calculation procedure, errors that could be introduced and methods to prevent their propagation. Based on that review, Boeing released the reference (b) Flight Operations Technical Bulletin and the reference (c) Aero Magazine article. These publications discuss the source of errors, steps taken by Boeing and steps available to operators to prevent the errors from occurring and propagating. Reference (b) is included with this letter and reference (c) was previously provided to the TAIC.
Discussion
Actions to reduce the occurrence of such of events will be most effective if they address all of the ways in which the error can occur. Among the incorrect takeoff speed events reported to Boeing, the Auckland event is unique in that the crew entered the correct weight figures in the FMC, but then overwrote the FMC speeds with manually calculated airport-analysis figures. In the other events reported to Boeing, the weight value entered into the FMC was incorrect, either because incorrect weight information was provided to the crew, or because the crew selected the inappropriate figure from the load sheet (e.g. selecting ZFW and entering it in the GW slot). The recommended software change to ensure that the speed and weight entries are not mismatched can identify the following two situations:
1. The crew enters the correct weight but overwrites the FMC-calculated speeds, or
2. The crew enters an incorrect weight and overwrites the FMC-calculated speeds with correctly calculated V speeds.
The recommended software check would be ineffective in preventing a large proportion of incorrect takeoff speed events - those in which an incorrect weight is entered into the FMC. Additionally, in the second situation cited above, the takeoff speeds are correct, but the recommended software change could reject or challenge them. Consideration must be given to the possibility that the crew might then elect to use the incorrect FMC-calculated speeds.
The takeoff speeds calculated by the FMC are balanced field length speeds which do not take into account the actual runway length, friction conditions, or specific techniques that take advantage of available runway length to gain improved climb performance. These factors are included in the airport analysis charts which crews use to manually calculate takeoff speeds. The effect of these factors can be significant. As an example, the following table lists takeoff speeds for a [Boeing] 737-700:
Model 737-700
Elevation 5330 ft
Runway Length 14,000 ft
Takeoff Weight 141,400 lbs
Temp 30C
Speed FMC Calculated(kts) Airport Analysis (Improved Climb)(kts)
V1 140 159
VR 141 162
V2 144 166
In the above example, the manually calculated speeds are approximately 20 knots faster than the FMC-calculated speeds. In the case of low runway friction, the manually calculated V1 speed can be up to 20 knots slower than FMS-calculated speeds. Thus, there are cases where the manually calculated speeds differ by more than a small percentage from the FMC-calculated speeds. The recommended software check would challenge or reject these valid entries creating nuisance warnings to the crew. Nuisance warnings reduce the effectiveness of a warning system and can defeat the original purpose of the warning system. We are, however, exploring the possibility of checking that the manually entered VR speed is not significantly lower than the FMC-calculated value. It appears that narrowing the check in this manner may produce the intended safety benefit while avoiding some of the problems mentioned above.
Boeing Action
Boeing will continue to examine the safety recommendation in the context of the broader issue regarding incorrect takeoff speeds. As the work progresses, we will determine whether changes to existing FMS installations may be warranted. Separately, we will also determine if such new features should be included in new FMS installations. At this point, no schedule has been set for the completion of our examination.
Background
This event is another example of incorrect takeoff speeds, which has previously been identified by Boeing as an issue for the industry. The common feature among these cases is that the takeoff speeds used by the crew are inappropriate to the specific operating conditions (actual weight, runway length, etc). The error or errors leading to the incorrect speed can happen at various points along the computational path, which consists of both manual and automated operations. In all cases, the results are the same - a takeoff is attempted with rotation at an inappropriate speed. The consequential risks to the airplane (tail strike, overweight takeoff, increased runway length, reduced manoeuvre margin to stall, reduced climb gradient, etc) are the same regardless of the specific error that led to the incorrect V speeds. Boeing is working to ensure that adequate and appropriate defences are in place to reduce the possibility that such errors are made or propagated.
Prior to the Auckland event, Boeing had reviewed the takeoff speed calculation procedure, errors that could be introduced and methods to prevent their propagation. Based on that review, Boeing released the reference (b) Flight Operations Technical Bulletin and the reference (c) Aero Magazine article. These publications discuss the source of errors, steps taken by Boeing and steps available to operators to prevent the errors from occurring and propagating. Reference (b) is included with this letter and reference (c) was previously provided to the TAIC.
Discussion
Actions to reduce the occurrence of such of events will be most effective if they address all of the ways in which the error can occur. Among the incorrect takeoff speed events reported to Boeing, the Auckland event is unique in that the crew entered the correct weight figures in the FMC, but then overwrote the FMC speeds with manually calculated airport-analysis figures. In the other events reported to Boeing, the weight value entered into the FMC was incorrect, either because incorrect weight information was provided to the crew, or because the crew selected the inappropriate figure from the load sheet (e.g. selecting ZFW and entering it in the GW slot). The recommended software change to ensure that the speed and weight entries are not mismatched can identify the following two situations:
1. The crew enters the correct weight but overwrites the FMC-calculated speeds, or
2. The crew enters an incorrect weight and overwrites the FMC-calculated speeds with correctly calculated V speeds.
The recommended software check would be ineffective in preventing a large proportion of incorrect takeoff speed events - those in which an incorrect weight is entered into the FMC. Additionally, in the second situation cited above, the takeoff speeds are correct, but the recommended software change could reject or challenge them. Consideration must be given to the possibility that the crew might then elect to use the incorrect FMC-calculated speeds.
The takeoff speeds calculated by the FMC are balanced field length speeds which do not take into account the actual runway length, friction conditions, or specific techniques that take advantage of available runway length to gain improved climb performance. These factors are included in the airport analysis charts which crews use to manually calculate takeoff speeds. The effect of these factors can be significant. As an example, the following table lists takeoff speeds for a [Boeing] 737-700:
Model 737-700
Elevation 5330 ft
Runway Length 14,000 ft
Takeoff Weight 141,400 lbs
Temp 30C
Speed FMC Calculated(kts) Airport Analysis (Improved Climb)(kts)
V1 140 159
VR 141 162
V2 144 166
In the above example, the manually calculated speeds are approximately 20 knots faster than the FMC-calculated speeds. In the case of low runway friction, the manually calculated V1 speed can be up to 20 knots slower than FMS-calculated speeds. Thus, there are cases where the manually calculated speeds differ by more than a small percentage from the FMC-calculated speeds. The recommended software check would challenge or reject these valid entries creating nuisance warnings to the crew. Nuisance warnings reduce the effectiveness of a warning system and can defeat the original purpose of the warning system. We are, however, exploring the possibility of checking that the manually entered VR speed is not significantly lower than the FMC-calculated value. It appears that narrowing the check in this manner may produce the intended safety benefit while avoiding some of the problems mentioned above.
Boeing Action
Boeing will continue to examine the safety recommendation in the context of the broader issue regarding incorrect takeoff speeds. As the work progresses, we will determine whether changes to existing FMS installations may be warranted. Separately, we will also determine if such new features should be included in new FMS installations. At this point, no schedule has been set for the completion of our examination.
Related Investigation(s)