radalt at high speed

[gravity32]

Early this year I started a discussion hoping to find out whether a commercial aircraft fitted with radalt would encounter any problems with height accuracy if flying at high speed. Given that radalt is most useful on approach for landing, when the plane would be going much slower than cruising speed, it seemed possible that there might be an upper speed limit to normal function.

On that occasion the commonsense view was that speed would not be relevant, however there was not a definitive answer. I now realize that there might be a better way to tackle the question.

This question relates to an aircraft which crashed and for which the FDR data is available. When the plane descended to about 2500 ft above ground the radalt started to record height. The height shown, plus ground elevation, corresponded well with altitude calculated from the pressure data. As the plane descended a slight divergence between the radalt and pressure became apparent, with pressure indicating a higher altitude than the radalt. Ground Proximity and Pull Up warnings were recorded. Toward the end the speed increased dramatically and the divergence also increased. The radalt data continued to the end.

If the radalt was not functioning for any reason, such as excessive speed, would the FDR file record a sign to say it was not operating? Perhaps No Computed Data (NCD)?

[skwinty]

I would think that the aircraft would have to be close to light speed before the radalt data started lagging the aircraft.

The radalt signal is already close to lightspeed so my opinion is the aircraft speed is irrelevant to the radalt accuracy.

[gravity32]

Thank you skwinty. Would you agree then that if anything upset the radalt that it would record NCD?

[skwinty]

If the radalt wasn't working, ie no output, the FDR should record that as an error from the instrument. The FDR would pick up the u/s flag.

If the radalt was just inaccurate then the FDR would record that as a valid reading.

It has been a long time since I worked in the field so take my statements with a pinch of salt.:)

[gravity32]

Inaccuracy is not really an issue at the end of the flight as an error of say 50% in radalt would only change the reading by a couple of feet. The important issue is whether one could be sure that, if the radalt was not working, it would display some indication such as NCD.

The concern is that the pressure altimeter is giving a final altitude 120 feet higher than the radalt. Some pilots are telling me that the pressure altimeter would be more reliable than the radalt.

Does anyone else have sure knowledge which confirms skwinty's answer?

[skwinty]

The pressure altitude is affected by, well, atmospheric pressure and that changes on a continual basis.

It will give accurate above mean sea level data whereas the radalt will give terrain dependant data relating to the distance between the aircraft and the terrain, so you should not expect them read them together unless you are investigating an accident.

[skwinty]

Reading your scenario again, you say the aircraft speed increased dramatically.

That would imply a severe nose down attitude and the signal from the radalt would have a longer distance to travel to the terrain thus causing the radalt to read a bit high.

The pressure changes the aircraft experiences in the sudden descent could also cause inaccurate pressure altitude data.

Hope this helps.:)

[gravity32]

Thanks skwinty. The altitude was calculated from the pressure recorded in the file, using the known barometric pressure and temperature on that day. The radalt and altimeter agreed when the plane was travelling at 312 knots at 2500 feet. They only began to differ when the plane descended and picked up speed, reaching 483 knots close to the ground, about 50 feet above sea level. It looks to me like a calibration error in the altimeter.

You don't happen to know the range of speeds the calibration should be good for at low altitude, and the permissible errors?

[skwinty]

You only calibrate your pressure altimeter on departure, cruise altitude and on arrival.

You ask ATC for QNH which is the current atmospheric pressure. A standard day at sea level is 29.92 inches of mercury, 1013.2 millibars, or 14.7 pounds per square inch.

[mono]

IMHO you are more likely to get hysteresis in a barometric measuring instrument than a RADALT, particularly with high rates of descent. Additionally an a/c flying outside of its normal envelope (and surely at almost 500kts near sea level this is the case) would possiblty suffer static source errors which were not correctly accounted for by the ADC's.

Certain ground conditions however are known to cause RADALT errors (slushy snow is one of them).

On another note NCD is just that. No Computed Data. If there were GPWS warnings then the rad alt was generating valid data (tho' it's possible it was erronous). An invalid rad alt means an inop GPWS.

[sycamore]

G32, every aircraft has a `Pressure error graph` for the ASI,and the Altimeter,calibrated during Flight tests,and should be in the Flight Manual. Thes cover the range of speeds/Mach numbers ,and airframe configurations,gear/flaps etc.
The radar altimeter will operate up to the aircrafts limiting altitude,but the actual gauge may only have a limited range,ie 0-5000`.It will show generally the actual height above the local terrain in S & L flight but will be in error at steep bank ,or pitch angles.It may be affected,ie lose-lock over some surfaces,ie sandy,or snow covered,or possibly over rough seas where the reflected `return signal can `disappear,or be `bounced away`,but that is radar altimeter type dependent.Anyway what type of aircraft are we discussing here..?

ed. In essence `speed` per se` will not affect the rad-alt,but it will affect the pressure altimeter/static source,if the aircraft is outside it`s normal operating envelope/configuration..

[winglit]

Would you happen to know at what attitude this aircraft was when it hit the ground? If it was nose down, rolling or even vertical, the rad alt reading would increase as the it only measures the ground beneath it.

IE the ground is perpendicular to the aircraft's longitudinal and lateral axis.

If you knew it's pitch and roll angle and the rad alt reading (slant range) a bit of trigonometry will give you corrected height.

Rad alts use a modulated continuous wave radar which is frequency modulated over a broad spectrum of frequencies. The distance measured depends on which frequency is returned. That means that it is not affected by doppler shift of the moving object.

[FlightPathOBN]

baro is temperature deviated, while radalt is not...

[gravity32]

skwinty,
The pressure data in the FDR file is unadjusted. When adjusted for known pressure and temperature on the day it showed the plane at the right altitude for the airport prior to take off and also at the right altitude according to radalt when flying at normal speed. So it appears the altimeter was not defective.

mono,
The plane had pretty well pulled out of its descent in the last few seconds so it seems hysteresis would not have been significant. It is likely that the discrepancy between radalt and pressure is due to the plane being outside its calibration envelope, as you suggest, and that unusual airflow around the static source would be suspected. The fact that GPWS was operating does imply the radalt was functioning.

winglit,
The plane was almost straight and level at the end so there would be no significant error in the radalt reading due to angles.

FlightPath OBN,
Temperature and baro were allowed for when calculating the pressure altitude.

sycamore,
Thanks for the info about the 'pressure error graph' for the ASI,and the altimeter. The plane in question was a Boeing 757. I do not have access to a manual. Is there any way to get hold of this graph?

[sycamore]

G32,you might ask on the tech log forum for 757 pilot input ..

[winlemony]

Hi all, I got some problem about the format to write the maintenance description in logbook. Here are some example:

1) PARTICIPATING IN CARRYING OUT AIRCRAFT JACKING FOR MAINTENANCE OPERATION AS PER AMM ATA CHAPTER 07-11-00-581-801-A, REVISION NO. 38 DATED JAN 01/2012. ALL SAFETY PROCEDURE AND PRECAUTION OBSERVED.

2) INSPECTION OF ENGINE COWLING CARRIED OUT I.A.W. AMM ATA 71. F.O.D. CHECK CARRIED OUT FOUND SATISFACTOY.

Note that I only include the words for task description, not the full content of logbook (a/c reg,date,etc).

I would be glad if somebody can comment whether my description words arrangement are good or somebody can give me an idea on how to write the task description.

For your information, I still under general training period until the end of October so mostly of my works are assisting, participating or observing, not carrying them out myself. I am going the pursue the Cat B1.1 license soon. TQ

:)

Regards.