How high was it flying?
Discussion
I heard a plane approaching, and looked out my window. An RAF A400M trundled past, about 50 yards from my house, and very low. ZM 405.
Flight radar said 250kts and 598 feet. It seemed less than that by maybe 100ft. I do see military stuff flying past from time to time, but not seen anything quite this low.
How accurate is the altitude from flight radar?
Flight radar said 250kts and 598 feet. It seemed less than that by maybe 100ft. I do see military stuff flying past from time to time, but not seen anything quite this low.
How accurate is the altitude from flight radar?
ADS-B packets report both barometric altitude at a standard pressure setting and GPS altitude, neither of which on their own are useful for determining really accurate height above terrain.
Bear in mind the design ethos is to have transponding aircraft use a standardised altitude model in order to avoid hitting each other, the logical extension of the Flight Level model wherein above a certain altitude you stop using variable altimeter settings geared to avoid hitting the ground, and go to a standard setting, again to avoid hitting each other.
You can usually find an ongoing minor bunfight on at least one of the aviation forums about the merits of using QFE (the pressure setting at which your altimeter would read zero if you were on the ground immediately below you) vs QNH (the pressure setting at which your altimeter would read zero if you were at mean sea level). If you're flying aerobatics, then it's always an opportunity to bring out this fantastic photo to demonstrate the merits of flying on the QFE vs the drawbacks of using the QNH.
Bear in mind the design ethos is to have transponding aircraft use a standardised altitude model in order to avoid hitting each other, the logical extension of the Flight Level model wherein above a certain altitude you stop using variable altimeter settings geared to avoid hitting the ground, and go to a standard setting, again to avoid hitting each other.
You can usually find an ongoing minor bunfight on at least one of the aviation forums about the merits of using QFE (the pressure setting at which your altimeter would read zero if you were on the ground immediately below you) vs QNH (the pressure setting at which your altimeter would read zero if you were at mean sea level). If you're flying aerobatics, then it's always an opportunity to bring out this fantastic photo to demonstrate the merits of flying on the QFE vs the drawbacks of using the QNH.
If on a low level training sortie, we are authorised down to 250ft MSD (minimum separation distance) which is judged visually and backed up by a radar altimeter set 10% lower ie 225ft. The baro altimeters will remain on QNH so will fluctuate with the terrain.
So the altitude reported by the aircraft via ADSB to FlightRadar will be a snapshot of the ground elevation + a minimum of 250ft.
So the altitude reported by the aircraft via ADSB to FlightRadar will be a snapshot of the ground elevation + a minimum of 250ft.
AndrewGP said:
If on a low level training sortie, we are authorised down to 250ft MSD (minimum separation distance) which is judged visually and backed up by a radar altimeter set 10% lower ie 225ft. The baro altimeters will remain on QNH so will fluctuate with the terrain.
So the altitude reported by the aircraft via ADSB to FlightRadar will be a snapshot of the ground elevation + a minimum of 250ft.
Thanks, So the altitude reported by the aircraft via ADSB to FlightRadar will be a snapshot of the ground elevation + a minimum of 250ft.
It was a most impressive sight
MrBig said:
ATG said:
holy cow ... that is an astonishing photo
Indeed!! Can anyone link to the back story behind it please?Some video:
Swindon/Fairford way the other night? A400m and a C17 flying low in formation, with around 6-7 loops over the area over 30 mins or so. First pass was really low, then subsequent ones were higher.
rooftop shot first
rooftop shot first
Edited by Byker28i on Tuesday 17th May 10:31
Edited by Byker28i on Tuesday 17th May 10:31
IanH755 said:
AndrewGP said:
The baro altimeters will remain on QNH so will fluctuate with the terrain.
Are you sure it will do that or is this a simple mix up between Baro and Rad?Assuming you can fly and accurately maintain a constant 250ft separation altitude from the terrain, and the terrain changes elevation, then the radalt will remain constant (showing the 250ft seperation) and the baro altimeter will change it's value as the terrain rises or falls. ie it will read whatever the terrain elevation is plus the 250ft.
AndrewGP said:
No mix up, may be poor wording on my part though!
Assuming you can fly and accurately maintain a constant 250ft separation altitude from the terrain, and the terrain changes elevation, then the radalt will remain constant (showing the 250ft seperation) and the baro altimeter will change it's value as the terrain rises or falls. ie it will read whatever the terrain elevation is plus the 250ft.
Ah I understand now, I didn't realise you were matching the terrain to maintain 250ft AGL, I thought you were sticking level against QNH as long as it didn't dip below 250ft AGL, my misunderstanding there.Assuming you can fly and accurately maintain a constant 250ft separation altitude from the terrain, and the terrain changes elevation, then the radalt will remain constant (showing the 250ft seperation) and the baro altimeter will change it's value as the terrain rises or falls. ie it will read whatever the terrain elevation is plus the 250ft.
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