Originally Posted by
deltafox44
Altitude data are not 100% reliable either. It is between 7200 and 7250 ft during takeoff roll then descend to 7075 ft after rotation (is there a hole in the runway ?)
Vertical speed is coherent with altitude variation. In 60 seconds between 05:38:39 and 05:39:39 altitude varies from 7200 to 8000 ft (800 ft/mn) and I computed the mean value of VS which is 822 ft/mn.
Of course, VS is very unstable and we don't know which part is due to the instrument errors and which (if any) to piloting or troubles with the controls...
On almost all air data systems and on all direct reading static systems, there is a difference in the static measurement with attitude, so the aircraft at approximately zero pitch attitude has a different sensed altitude to being at liftoff attitude. On Boeing and Airbus data analysis, you will usually note a slight dip in recorded altitude as the aircraft obtains liftoff attitude. The radar altitude also has a fixed offset that is based on approximately the attitude on an approach, so it will tend to under read on the first part of a takeoff, and over read at the liftoff attitude. Both systems could be corrected to give accurate readings at all times, for the reference point on the static and wheel height for radar altitude, but that has not been done on Boeings from the 737-777 (haven't looked at 78 datasets) and the A320-330-340 (don't know about the 350,380)
It is also possible the runways are not level, like NFFN RWY02.