Radar Alt versus Baro Alt
Here is a picture of my HUD and the jet flying over water.
Why does the Radar Altimeter (7030) show me higher than the Baro Altimeter (6060)?
OldGoat5 last edited by
Incorrectly set altimeter, ask tower or your agency for qnh.
Eagle-Eye last edited by
This post is deleted!
l3crusader last edited by
It might also happen on the correct QNH, if temperature is significantly higher than the standard atmosphere model temperature of 15°C at sea level.
I contacted the tower and changed the QNH like OldGoat said, but no change in altimeter readings. It must have been like l3crusader said, sea level temp is higher than 15° C . Thanks guys. So I just checked Kunsan weather and currently it’s 27.7° C and 29.87 in pressure.
But then I got this about QNH from Wikipeida. “An airfield QNH will cause the altimeter to show airfield altitude, that is, the altitude of the centre point of the main runway above sea level on landing, irrespective of the temperature.” What do you say, l3crusader?
l3crusader last edited by
If this stuff interests you, there is the article on the atmospheric model
As you probably know already, the baro altimeter is basically a pressure sensor - it just gives you the altitude you would have in the standard atmosphere with the same pressure. QNH settings allows to modify the reading to get a more accurate altitude given the QNH, but since the pressure curve with altitude is dependent upon both pressure at sea level and temperature in all layers, it cannot be 100% accurate all the time either.
Thanks for the article/attachment. It said that BMS models the weather and time of day. I couldn’t tell from the articles how much temperature would affect the altitude reading. I probably missed the explanation somewhere in there. But there is a significant difference between the altimeters, 970 feet (7030-6060). Now if I want to make an escape with a Split-S, I normally would have relied on the Radar Altimeter, but in this situation the radar showed I had 970 more feet below sea level. That could mean a difference in speed of 100 knots or more. If I relied on that radar altimeter without slowing down, I could crash. In RL and BMS should I just rely on the altimeter with the lowest altitude for Split-S maneuvers, or any other maneuver?
Frederf last edited by
Barometric altimetry makes a lot of assumptions about other atmospheric values to make a altitude readout by measuring the local pressure. Sure it has an altimeter setting knob but that’s only a one dimensional calibration and it works just fine in BMS that given the QNH setting you get aerodrome elevation when at the aerodrome. That’s the only altitude that’s guaranteed to be 100% accurate with QNH.
However all other altitudes rely on the curve built into the instrument to match the pressure curve of the real environment. Even calibrated to cross at the QNH point properly it’s probable that there will be some error and that the error will be proportional to the deviation from the calibrated elevation. Higher altitudes are much more s
In BMS the default weather values are a little bonkers and it doesn’t handle some aspects of weather that well. I have found that extremely high or low pressures result in poor matching of true altitude to indicated altitude even with the correct altimeter setting. My personal fix is to limit the max/min pressure range to a more realistic and well-behaved range. Take Seoul’s forecast to see a real short term variability of pressures ( http://www.wunderground.com/kr/seoul ) about 29.7 to 30.0" Hg. If we constrain pressures to a 0.3" margin around standard we find that the F-16’s altimeter has minimal error vs true altitude. The bonkers “world record” type QNHs that we get routinely test the limits of the altimeter’s ability to successfully correlate pressure to altitude accurately.