I work in flight data analytics, we are trying to find a robust way to detect liftoff

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I work for a flight data analytics company, we are trying to find a robust way to detect the exact point of liftoff of an aircraft using data available from a flight recorderCurrently, we use a mix of gear, pitch, longitudinal / normal acceleration, and altitude indications.

Suggestions, brainstorming, questions... All is welcome!

[herostratus]

Umm, WoW switch normally inputs to the FDR does it not?

[MechEngr]

Not an FDR input, but would be monitored by an anti-lock system: at the exact point of liftoff the tires will instantaneously begin to slow in rotation at a linear rate. You will have a separate lift-off for each independent axle. If there is a bounce the speed will suddenly increase to flight speed. Even if the data rate for the rotation monitoring isn't high, one can interpolate both the on-ground speed and the airborne speed and see where they intersect.

WOW may indicate before the last contact with the ground has occurred and there won't be a way to determine exactly when except that it happened between two recorded times. WOW, OTOH, is almost certainly recorded in any aircraft that depends on WOW for automation.

[Uplinker]

Main gear tyre pressures - once all have gone to a minimum, the aircraft must have no weight on any tyre and therefore be airborne ?

But these data are not universally measured on all aircraft, and I don't know if they are recorded on the flight recorder ?

[TURIN]

Aren't the WOW sensors linked to thrust reverser and ground spoiler activation too? Surely there's a discrete input to the EEC (FADEC) and flight control system logic.

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Umm, WoW switch normally inputs to the FDR does it not?
Only new aircraft like the a350 and 787 record things like WoW and wheel RPM

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I can only use data provided by the FDR. WOW and OTOH are not an option unless its a new aircraft like the 787 or 350

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Tyre pressures are not recorded by any FDR that i have come across besides the 787 and 350 but they record things such as wheel RPM which is more useful in the context of my post

[Uplinker]

In that case; what about Rad Alt; corrected for pitch attitude?

The measured Rad Alt will reduce to a minimum as the aircraft is pitched up, (Rad Alt antennas usually fitted underneath the rear fuselage), then after a pause, the Rad Alt will increase as the aircraft lifts up.

I am assuming that Rad Alt will be accurate enough when the antennas are within a meter or so from the ground, but only a positive or negative trend will need to be registered.

And there will need to be constant reference to the aircraft pitch angle, used to correct the measured radio altitude height; since pitch angle will change the measured reading.

[ahwalk01]

The type of aircraft would really help here