Residual Groundspeed

[Fpetrowitsch]

Hi everybody, does anybody knows exactly what is residual ground speed from IRS and why does it, thanks and greetings from chile.

[BOAC]

If, for any reason. the platform has developed a tilt (real or 'electronic' in the case of strap-down systems) away from true vertical. the accelerometers may detect such a tilt as an acceleration and integrate this to produce a groundspeed. Any 'residual' outside stated tolerances might indicate a platform problem. The highest I have seen is on a 737-300 on arrival on stand - 118kts - and, no, I am not prone to taxying on at that speed:). This was caused by the fitment of a non-approved IRS platform by the previous lessor.

[Fpetrowitsch]

Thankyou Mr. BOAC, by the way did you flew in BOAC airline? Well thanks again for your comment about my thread, you are very kind, good luck.

[NSEU]

The IRS measures aircraft acceleration and senses attitude throughout the flight to compute groundspeed, aircraft track and, of course, position.

i.e. The aircraft begins at zero knots at the departure gate at a known position. When the aircraft starts moving, groundspeed is computed from the sums of all major and minor accelerations in all directions (over the duration of the flight).

Unfortunately, the system is not 100% perfect, so tiny errors in the measuring system accumulate over time. This may result in the IRS thinking that it is still moving at the end of a flight and/or the IRU thinking it is not where it really is.

Residual groundspeed is the speed error recorded at the end of a flight (aircraft not moving). If the error is too large, then there may be a problem with the IRS.

Your instrument displays may show this residual groundspeed error, especially on aircraft which have no GPS input into the Flight Management system.

Rgds
NSEU

[Microburst2002]

In addition so the above said

The IRS position is not 100% perfect, and therefore, the gravity force sensed by the accelerometers is not fully accounted as such. A small part of it is wrongly counted as horizontal acceleration. Therefore, there is a GS error. And this error increases the position error, which in turn changes the GS error, and so on. But this error is oscillatory, and the sensed positon will wander about the real position.

You can see in the ND how the airplane positon changes slowly with respect to the runway when you are on the parking stand, when the residual GS is high.

Also, when you are taxiing, you can sometimes feel you are very fast but the GS is not so fast. or the opposite. Crosscheck with the other pilot GS. Many times there is a great difference between both.

This GS drift affects the FPV, too. Sometimes while flying an ILS you can notice that the track "green diamond" is not perfecly aligned with the ILS course, even if the AP is perfectly maintaining the LOC. This happens when there is a large GS drift at right angles with the LOC course.