Cross-Wind direction on a take-off roll

[andrej]

Hello Hardy,
Recently, I came across following situations:

1.Departure from EGLL - RWY 09R
METAR: EGLL 271420Z AUTO 06008KT 020V100 9999 BKN017 BKN021 11/07 Q1003
On a take-off roll, at 100kts the wind direction appeared on a ND and it indicated direction right on runway heading (089*), while getting airborne (50 feet upwards), the cross-wind was noticed on ND. In this case, I know that Wind was variable between 020* and 100*, so it is possible, that at a certain point it could actually be right direction. But the sudden change to 060* area, made me wonder.
SITU File: https://www.hoppie.nl/forum/var/2024-04-28_08.45.10_Departure_from_EGLL.situ

2.Departure from EDDK - RWY 13L
METAR: EDDK 271220Z AUTO 17013KT CAVOK 17/08 Q1006 NOSIG
On a take-off roll, the wind direction at 100 kts was right on a runway heading (134*), while getting airborne (50 feet upwards), the cross-wind was noticed (on ND).
SITU File: https://www.hoppie.nl/forum/var/2024-04-28_08.43.05_Departure_from_EDDK.situ

I know that two samples are not enough to make any conclusion, but I find it interesting that on both occasions, the wind direction while on a take-off roll, were straight headwinds. As I was getting airborne, the cross-wind (reported wind direction appeared). I am always using live weather (download METARs from Internet option). Both situations are attached.

Thank you,
Andrej

[Hardy Heinlin]

Hello Andrej,

this effect is intentional because it reflects the realistic behaviour of the IRS gyro and accelerometer systems.

The wind direction shown on the ND and in the FMC is computed by the IRS (which also uses the TAS from the ADCs).

You probably know the wind vector triangle which consists of:
Vector 1: Aircraft heading and TAS
Vector 2: Aircraft track and groundspeed
Vector 3: Wind direction and wind speed

If 2 of these 3 vectors are known, the unknown 3rd vector can be computed. E.g. during flight planning, when you have the wind info from the office, the route track from the chart and the desired groundspeed, you can compute the required heading and TAS to maintain that route track. Here, the heading and TAS are the unknown 3rd vector.

The aircraft systems, on the other hand, know the heading/TAS vector and the track/groundspeed vector; those systems want to compute the wind vector. There, the wind is the unknown 3rd vector.

The compass gyro provides the sensed heading, the ADC provides the TAS, the IRS motion sensor system provides the sensed track and groundspeed.

The wind vector can not be sensed. Even if there were a windsock on the aircraft, the wind couldn't be detected as the aircraft moves with the wind like a balloon. So the wind vector needs to be computed, not sensed. For the computation, it needs reasonable heading, TAS, track, and groundspeed data.

Now, on the ground, heading and track are nearly equal, depending on the wheel friction. There may be a real crosswind component, but the aircraft's heading and track are almost equal, therefore the system computes a crosswind component that is nearly zero. It just sees a headwind component, if there is one.

In flight, in crosswind conditions, heading and track will be different because the wheels are off the ground and the aircraft can freely move with the wind.

(Also restricted on the ground is the tailwind detection as the pitot tubes are directed forward. Airspeeds are never negative. Only when the ADC TAS is above zero, the system can know that the aircraft is moving through the air mass. E.g. when the groundspeed is 35 and ADC TAS is 30, the computed tailwind is 5. Say, when the groundspeed is 35 and ADC TAS is 0, the computed tailwind is not necessarily 35; it could be anything since the pitot tubes cannot see backwards. The whole negative number area is unkown; the system can only work when the numbers get positive.)


Regards,

|-|ardy

[andrej]

Hello Hardy,
thank you for very informative and detailed explanation.

Sincerely,
Andrej