Зачем - могу только догадываться, утверждать ничего не буду.
...
Я тоже ничего не утверждаю, только предполагаю, что такие изменения воздушного потока вызваны "чрезмерно активным рулением" . Выделил бы фразу -" ни в одном руководстве не описано, как использовать эти данные при взлете". Остальное (если есть желание) по-переводите.
At around 140 knots the pilot flying observed a speed trend vector in the negative direction. The
captain stated that at the same time he observed a large trend vector in the positive direction. The
captain did not find this discrepancy an issue and made no comments about this.
The captain as the pilot monitoring should be observing and verifying conditions of aircraft systems
during the takeoff roll. The captain had noticed a large positive speed trend vector but deemed this
not an issue for takeoff. The first officer also observed an irregular speed trend vector during the
takeoff roll. This implies that the instruments in cockpit were showing large deviations which were
noticed by both crewmembers.
The speed trend vector is intended to assist the crew in-flight with throttle selection to fly
appropriate airspeeds. There is no reference in any manual or training program as to how the speed
trend information should be used or monitored during takeoff. The speed trend vector is calculated
using the computed airspeed and the aircraft longitudinal acceleration. According to the data from
the flight data recorder the longitudinal acceleration was smooth. The flight data shows that speed
variations began appearing in the data at 110 knots of airspeed. The recorded computed airspeed,
by contrast, showed sharp increases and decreases around 135 knots. Calculations based on the
available recorded airspeed data showed that the speed trend arrow could have increased to the full
length of the display in positive direction. Also, the later steep decrease in the recorded airspeed,
may have caused a negative speed trend vector. The recorded computed airspeed is available from
the captain’s side only; no data is available from the first officer’s side because it is not recorded.
The observations made by both crew members regarding the speed trend vector being large
positive and negative could be explained by a difference in measured airspeeds between the left
and right side airspeed computers (ADIRU) of the aircraft. Analysis of the angle of attack sensor,
which measures the airflow direction passing over the aircraft, showed that there were differences
between the left and right side. This angle of attack difference leads to the conclusion that the
airflow was disturbed and the airflow was asymmetric between the left and right side. In any event
when airspeed or angle of attack deviations are large for a certain period, warnings will appear
. Such warnings were not noted by the crew or recorded. Analyses show that the period where
large changes occurred lasted for a short time. It is thus very likely the differences would not have
triggered warnings.