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Use of Erroneous Parameters at Take-Off

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Category: Runway Excursion Runway Excursion
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Description

The use of erroneous take-off parameters (thrust and speeds), usually as a result of using incorrect values for take off weight in performance calculations, can result in early rotation with tail strike, loss of control when airborne, or overrun as a result of failure to get airborne.

Identifying Common Causes

In 2007, following the investigation of two serious incidents involving tail strikes that had occurred at Paris, Charles de Gaull Airport (LFPG), a study entitled "USE OF ERRONEOUS PARAMETERS AT TAKEOFF" was conducted by the Applied Anthropology Laboratory (LAA) at the request of Le Bureau d'Enquêtes et d'Analyses (BEA) and the French Civil Aviation Authority (DGAC).

Various investigation bodies, airlines and manufacturers were consulted in the course of the study because several other accidents, serious incidents and incidents of the same type have occurred around the world during recent years. These generally involved new generation aircraft, being caused by more or less significant errors in entering takeoff parameters that were not detected by crews. The errors occurred in various airlines and on various types of large aircraft manufactured by Airbus and Boeing. The most serious event occurred in 2004 and involved the destruction of a B747-200 Cargo on takeoff at Halifax and the death of all the crew members. Other incidents arising from errors of the same type, but of lesser magnitude, were reported more recently on latest-generation large and medium-sized aircraft such as Embraer 190.

The complete text of the study is accessible from (See: Further Reading).

Conclusion of the Study

In conclusion, the research identified the following problematic issues:

  • The variety of events shows that the problem of determining and using takeoff parameters is independent of the operating airline, aircraft type, equipment and method used,
  • Errors relating to takeoff data are frequent. They are generally detected by application of airline operating modes or by personal methods such as mental calculation,
  • Studied cases reveal that failures correspond to the "calculation of takeoff parameters" and "input of speeds into the FMS" functions, but do not correspond to errors in the "weight input into the FMS" function,
  • In several cases, the Zero Fuel Weight (ZFW) was entered instead of the Take-off Weight (TOW) into the performance calculator,
  • Half of the crews who responded to the survey carried out in one of the participating airlines had experienced errors in parameters or configuration at takeoff, some of which involved the weight input into the FMS,
  • Pilots' knowledge of the order of magnitude of these parameter values, determined by empirical methods, is the most frequently cited strategy used to avoid significant errors,
  • Input of the weight used in parameter calculation, in whatever medium it may be (by ACARS, in a computer, manually), is one of the determining steps in the process of takeoff preparation. It is this, by affecting both the thrust and the speeds, that determines takeoff safety,
  • The real-time availability of final weight information shortly before departure requires the crew to perform a large number of tasks, inputs and parameter displays under strong time pressure,
  • Checks on the "takeoff parameter calculation" function can be shown to be ineffective because they consist of verifying the input of the value but not the accuracy of the value itself,
  • In the same way, the check of data featuring on several media often proves to be ineffective. It is often limited to item by item comparisons. If the item is wrong, the check is correct but inadequate because it doesn't cover overall consistency. In particular, there is no comparison between values for takeoff weight given in the final loadsheet, on the takeoff paper or electronic "card" and in the FMS,
  • The reference speed values suggested by some FMS can be easily changed. They do not enable routine detection of prior calculation errors,
  • Studied FMS allow insertion of weight and speed values that are inconsistent or outside the operational limits of the aircraft concerned. Some accept an omission to enter speeds without the crew being alerted,
  • The weight values manipulated by crews before the flight can appear, depending on the documents or software, under various names or acronyms and in different units and formats for the same data, which makes them too difficult to memorise,
  • Time pressure and task interruptions are frequently cited in surveys as common factors contributing to errors. The observations showed that the crews' work load increases as the departure time approaches and that the normal operation actions of the captain were all the more disrupted,
  • During the takeoff run, the possible decision to reject takeoff based on an erroneous V1 no longer guarantees safety margins,
  • On cockpit display screens of the PFD-type (Primary Flight Display), the marker representing Vr is not displayed at low speed. Further, it can be difficult to distinguish it from the marker representing V1, especially when the two values are similar.
  • In several cases, crews perceived abnormal airplane behaviour during takeoff. Some took off “normally”, i.e. no abnormal behaviour counter strategy was applied. Others were able to adopt different strategies: stopping takeoff, increasing thrust, delayed rotation.

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