Airbus EGPWS ND lower altitude values
This is straight from Honeywell and it makes more sense than Airbus' awful FCOM description. However, the delights of long haul to Jo’burg have allowed me a while longer to study this reply and I’m now firmly of the opinion that the riddle is yet to be solved. The outstanding question remains: what use is the lower terrain value to a pilot?
Q: How is the lower three-digit number calculated?
A: This is a bit complicated. The lower number is ALWAYS the lowest elevation of displayed terrain (or obstacle, if Obstacle Alerting is enabled) in the displayed range, as determined by the EGPWS on-board terrain database. The lowest displayed terrain depends on where the aircraft is (on ground, in cruise) and whether the aircraft is ascending to or descending from cruise elevation. On ground, the lowest displayed terrain is governed by the display cutoff, 400 feet above runway elevation. This cutoff allows the pilot to land without the terrain near the airport appearing as yellow (a threat). At cruise altitude, the lowest displayed terrain follows an algorithm that it is approximately 50% of the highest displayed terrain. This is done to de-clutter the display, and to show the pilot only the terrain that is the most immediate threat to the aircraft. If all terrain (down to sea level) were displayed, the entire display would be some pattern of green at cruise altitude and quickly spotting the highest terrain would not be easy. As the aircraft climbs above 2400 feet above runway elevation, the lowest displayed terrain will be 2000 feet below the aircraft until the aircraft has climbed above an altitude more that 2000 feet above the 50% algorithm value, at which time the lowest displayed terrain will remain at the 50% value. As the aircraft descends from cruise, the lowest displayed terrain will be the 50% value until the aircraft has descend to within 2000 feet above the 50% value. From this point downward, the lowest displayed terrain will be 2000 feet below the aircraft until the aircraft reaches 2400 feet above destination runway altitude. Further descent to the runway will result in the lowest terrain displayed being a minimum of 400 feet above runway elevation. See, I told you it was complicated!
Unfortunately the Honeywell answer either doesn’t work or is so approx as to be rendered useless. I’ll show you using the 5 original real world screen shots from our A330E; below.
I have no issues with image 1 – on the ground in HKG. This works and I can (almost) see the logic of limiting the lowest value to 400ft though I can think of enough incidents to make this idea flawed. Furthermore looking at the fifth image, you can see the lower value at just one hundred feet so that blows Honeywell's theory that the minimum value is 400ft!
Moving on to 2 at around 5000ft, heading south between HK Island and Lantau. The upper value shows 030 green which makes sense for the terrain on Lantau and is green, the aircraft being above it. But the lower value remains a mystery. The Honeywell explanation suggests that this value remains 004 until the aircraft is at 2400ft then reads 2000ft below the aircraft (5000-2000=3000) until 2000ft above the 50% value (3000*.5=1500) or 3500ft. Thus 5000ft being above 3500ft, this lower value should read the 50% value of 015. Yet it remains 004.
Moving quickly on to 3 and 4 at cruise altitude. Image 3 is approaching Taiwan (RCTP) with the ridge of high ground down the Island and an expected high highest value. The Honeywell info again suggests being at high altitude with the gear up, I would expect to see the 50% value of 066. Yet we see 038 which is just 29% of 132; not even very approximately 50%. Finally image 4 approaching Cheju (RKPC) shows a highest value of 064 suggesting a lower value of 50% = 032 yet again we see 015, merely 23%!
It would be interesting to see a Honeywell prediction of this info in the final few minutes of (say) the Cali flight. I wonder how useful it would have been? Instead of confusion over the tuning of an NDB, the conversation might have hinged on the lower terrain value. This displayed piece of data is about as useless and inaccurate as the FMGC PROG page VDEV indication!
The moment I get better, more accurate info: I'll post it.
Last updated: 11:05 13 May 2008