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TAWS Premature descent alert: when should it trigger if low ?

Hello everyone,

Just as an experiment, we tried to trigger a specific algorithm of the TAWS: the Premature Descent Alert.


We were configured for the ILS 17 Y in LFMV (Avignon).
During the procedure turn, we continued the descent from expected 1700ft down to 1350-1400ft (QNH was set up properly :-)).The glideslope was captured from this altitude at 4nm (instead of 5nm in the procedure).
Terrain is flat there so, there was no reason the main TAWS algorithm (Forward Looking Terrain Avoidance = FLTA) would trigger.

Result of the experiment: we heard no alert, although well below expected altitude before FAF.

I wondered if TAWS experts could speak about “ILS protection” function.

And for the information, ILS in LFMV will be withdrawn by the end of october.

Once you were on the glide path did you stay on the glide path or continue your descent 200ft below it?

France

You have to be very bellow the glide path or flying onwrong signal for it to work no? I doubt there is an ILS where terrain sits in the 200ft/15nm or 0.7" envelope bellow the glide path? For non-precision approaches yes, you can find more scenarios

Last Edited by Ibra at 18 Sep 07:06
Paris/Essex, France/UK, United Kingdom

Good question
Once glide was intercepted, we followed it

If you followed the glideslope then surely the TAWS would not sound until you were somewhere close to the flare. At least that’s AIUI.

France

I doubt there is an ILS where terrain sits in the 200ft/15nm or 0.7" envelope bellow the glide path?

It is not really about terrain hazard, rather an alert that you deviating from the procedure.

If my calculation is right, 0,7 degree 5nm from threshold, gives 370ft. Maybe we were not low enough. However, the algorithm (the schematics is from Garmin) suggests alert if 200ft below published altitude before FAF. Maybe @NCYankee has an idea ?

This was just for talk !

What was the air temperature at the airport where QNH was determined? The baro altimeter indicates lower than the true altitude when the temperature is warmer than standard.15 degrees C. The TAWS-B is based on GPS altitude and unaffected by temperature, so on a particularly warm day, the altimeter will read lower than the GPS altitude, assuming the correct QNH value is entered into the altimeter. You might want to monitor the GPS altitude (GSL) shown on the GTN750 terrain view as it is what will be used to determine the height at which the alarm takes place.

I calculate the difference between a 3 degree slope and the 2.3 degree slope (-0.7) at 5 NM to be 352 feet.

Last Edited by NCYankee at 18 Sep 18:40
KUZA, United States

Hello NYC !
I had that in mind too: it was 32 degrees on the ground.
I didn’t check the GPS altitude unfortunately hence cannot give the answer. I could retrieve the logs to check it.

That temperature would account for approximately 100 foot difference between a nominal altimeter and the true height at the elevations you were discussing.

KUZA, United States
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