QAR/FDM and inaccurate data.
 

Consider this -
A pilot receives a call from his FDM department because he has exceeded a speed parameter such as tyre speed by 5kt.. The source of the data is IRS 1 providing ground speed ,but the IRS has been aligned at the time of the incident for 12 hours and has built up some errors so that it is overstating the perceived ground speed. and has caused the erroneous trigger. How would a pilot prove that he has been incorrectly caught by the system?

5 kts seems to be an awful lot of speed error for an IRS to me.
Is there also position data available from IRS 1? Does this position (and the change thereof) reflect the position of the aircraft and can the shape of the taxi path be recognized? If a large speed bias is indeed present, the shape of the taxi path as represented by the IRS 1 position must be misformed.
What was the IRS 1 speed when the aircraft was static (e.g. parked at the gate)?
Is there other positition data available on the QAR, e.g. from the GPS?
Or if you are really persistent: are there recordings from ADS-B / MLAT / SMCGS.

Groundspeed can be derived quite accurately by fitting a curve through position reports (given that the position reports are quite accurate).

ATCast for my aircraft the ground speed difference can be up to 15kt before the IRS is considered unservicable.. As you say an interpolation of the speed when the aircraft was static and when the event occurred could show what the error was. As to your last I doubt the environment where this happened would have any of those systems.

Normally (or in an ideal world?), the pilot shouldn't have to prove anything. FDM should be non-punitive. It's purpose is not to "catch crew", but rather find out what went wrong. Here in Europe, EU-OPS 1.037 makes that an explicit requirement.

From a safety standpoint, the question is rather why the plane was operated so closely to and possibly above the max tire speed.

My aircraft is a non-fbw widebody airbus. The incident took place at a hot and high airfield where the only performance figures given are for a slat only take off.. Airbus in their performance figures gives limit codes and in this case they were 2nd segment climb and tyre speed.Given the airfield altitude and a nominal temperature and calculating the TAS they give a 5kt margin on exceeding the max tyre speed at the IAS Vr. As a fleet we do not perform many slat only take-offs and Airbus admit that there is an increased risk of tail strike in this config so there may be a slower rotation rate leading to a higher unstick speed,but as you say this is too close to a potentially critical limit.

As has been said, the pilot should not have to prove anything. The Flight Safety department will have enough data available, both recorded technical data and airfield environmental data, to make a reasonable assessment of what occurred. They should also be able to asses the accuracy of the IRS derived ground speed. The pilot should also be given access to this data and make his/her own assessment.

The FS department's only interest should be what, why and do we need to adjust our procedures or training or both.

From a technical point of view I don't see why one would trust a system that allows for a 15 kts tolerance to trigger alerts that require 5 kts or less accuracy. The FDM department should recognize there is a flaw in this logic. If the trigger level is max tire speed +5kts, then in theory an alert could be triggered at 10 kts below max tire speed. On the other extreme, one could exceed the max tire speed by 20 kts before the alert is triggered.

From a safety viewpoint I agree with Cpt_Schmerzfrei; the focus should be on why the plane was operated close to or beyond it's limits.

In your explanation on the calculation of the take off performance you mention altitude, temperature, TAS and IAS. To come to groundspeed, wind plays a role as well. Is wind taken into account in calculating the tire speed? If so, and if the actual head wind component was lower than expected, or a tailwind was encountered, the 5 kt margin given by the performance figures disappears like snow on a hot runway.

... and this is exactly what your FDM program manager / safety department is (or should be) interested in: Find situations that pose a hazard that needs to be mitigated, and not to find proof that Cpt. X is a reckless pilot who needs to get fired and/or seriously spanked. You might even want to think about filing a safety report addressing this very issue. :ok:

Thank you all for the comments.Unfortunately I think my company is still in a reactive rather than a proactive mode but they have changed the performance at that airfield.

What if this was a genuine catch? And let's say that the company SOP's and performance calculations were followed to the letter. All it proves is that the company's figures may need revising and/or that duff data is used for FDM purposes. Therefore probably lots of other duff calls have been made.

Hopefully, evaluating tyre speeds to within +/- 5 kts is not something that any operator should expect their pilots to perform on a take-off roll.

PM