Altimeter query.
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Hello Forum.
When flying over the sea, should both the barometric and radar altimeter show the same height?
When performing a night / low visibility ILS landing, flying an arc DME, it’s possible to wander to the right or left of the arc. With high ground in the vicinity, should one use the radar altimeter in preference to the barometric version?
Thanks,
Bayonet.
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if you set the Barometer (I guess that’s what you mean by altimeter) with the right pressure (QNH), they will show the same height over the sea.
You can also use QFE over an airstrip, then you shall see also the same height.Airfieldcharts have altitude perimeters and special notifications for things like mountains, towers, cities etc.
The charts shall be your way to go with barometric measure, Radar Altitude is an additional help, but not the thing you should rely on primarily.That’s at least what I think - let’s wait for more experienced people
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On my phone right now, so can’t really type out an extensive reply, but everything you need to know about altimetry should be in this document: https://www.dropbox.com/s/wrrkcb7nfa0qbpp/Altimetry.pdf?dl=0
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The only guaranteed reading is that at airfield altimeter will read airfield elevation for a QNH setting. At any other altitude it assumes a particular pressure lapse rate. Default BMS weather has really weird trends vertically and the altimeter will not show a very accurate high altitude reading. Use downloaded real world weather and the errors will be much smaller.
A well-designed approach will have adequate terrain clearance for the given altitude laterally and vertically. The US TERPS or PAN-OPS approach design books make interesting reading to get a feeling for how far off approach you can get before it’s time to abort to MSA. The barometric altitude is the primary minimum height reference in all approaches I have seen but a radio height crosscheck is wise.
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With high ground in the vicinity, should one use the radar altimeter in preference to the barometric version?
I would’ve thought THIS was the main purpose of the radar altimeter.
You’re on your attack run on an airbase high up in the mountains… do you call the tower first to get a proper altimeter setting? No. You just come in, rely on your radar altimeter, bomb the airbase, and get out
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Thanks Ice.
For clarity, the context here was a low viz ILS approach to home plate and whether the radar altimeter should be used in “preference” to the barometric altimeter where high ground is a factor.
Your description of a bombing run is, of course, correct.
Bayonet.
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You’re RTB but the base is under fog or it’s raining heavily so visibility is low. You get proper altimeter settings from tower so you know your exact altitude. You know your “path” home, but you know if you stray from the path, you’ll encounter high ground which has a known Pk of 100%. I’d say you use ALL the tools at your disposal to make sure you don’t meet Mother Earth too early. Err on the side of caution… if one says 1,500 ft but the other one says 200 ft, do you really want to risk it?
Anyway, correct me if I’m wrong, but shouldn’t you be flying at an altitude that clears all known nearby terrain anyway? What airbase was home plate?
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LowVis to unknown QFE (should’ve been known if RTB, but who bothers to take such a note during RAMP?
I sure wouldn’t)
My suggestion is to overfly the runway looking at RadarAlt and setting the BaroAlt to that very altitude (now you have QFE on your BaroAlt) and then keeping the RadarAlt as prime until final where the BaroAlt should matter just in case the runway has a terrain slope ahead of it (not too un-common). -
but in lower airspaces it is necessary all planes of the location use the same barometric pressure for division (while above specified height all planes have to use QNE).
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Hello Forum.
When performing a night / low visibility ILS landing, flying an arc DME, it’s possible to wander to the right or left of the arc. With high ground in the vicinity, should one use the radar altimeter in preference to the barometric version?
Thanks,
Bayonet.When you follow a terminal procedure to land (ILS or TACAN/VORTAC one) your ONLY way is the usage of barometric altitude with local QNH from the tower of the airport that you want to land. No room for argument here, IF we want to say “as real as it gets”!
Even if the airport is surrounding with high mountains or any other dangerous obstacles, the procedure created with them in consideration and has a completely safe path for all of its track!
Now if the desired airport has no published terminal procedure, this is an other story…
Last for the guys that may suggest also the usage of radar altimeter in an area with high obstacles (like mountains), they must think that sometimes the popography of an area has a very steep changes in the altitude of the terrain that leaves to the pilot a very small amount of time to react after the reading to the radar altimeter ;).
Nikos. -
Neystratiou is right. Fly the published approach and you are clear of obstacles. Radar altitude is the height you have now but does not predict the cliff you are flying towards. So radar altitude is a is a bad replacement of good planning. This is also the case (or especially) with a bombing run.
So bottum line: use baro with QNH and fly your planned flightpath.
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P.S. wrong name
Haha, no you were just fine.
Nikos is my name, and I like to use (and be used) my real name instead of my callsign :).
Nikos.
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Was not sure so I thought Oops
Good to know -
When you follow a terminal procedure to land (ILS or TACAN/VORTAC one) your ONLY way is the usage of barometric altitude with local QNH from the tower of the airport that you want to land. No room for argument here, IF we want to say “as real as it gets”!
Even if the airport is surrounding with high mountains or any other dangerous obstacles, the procedure created with them in consideration and has a completely safe path for all of its track!I agree with you 100% but his scenario had him straying from the published track… granted he didn’t mention how far he was straying but he didn’t mention which airbase as well.
If he can fly the track and stray maybe 1-1.5nm from the path, then he should be fine. If he strays further out and the radar altimeter is telling him something else, well, like I said, terra firma has a Pk of 100%! -
I agree with you 100% but his scenario had him straying from the published track… granted he didn’t mention how far he was straying but he didn’t mention which airbase as well.
If he can fly the track and stray maybe 1-1.5nm from the path, then he should be fine. If he strays further out and the radar altimeter is telling him something else, well, like I said, terra firma has a Pk of 100%!Well, again my dear friend you will NOT rely on the radar altimeter.
Your next step in the case that you are offset of the defined track of the procedure is to climb as soon as possible to the minimun safe/sector altitude, as defined in the chart.
Then if you have a human ATC controller you can be guided by him on your next action (to holding or to a new approach) or you will find the way (with safety in mind) to the holding first (assuming that your fuel let you do this) and then with checking the nearby traffic and informing at the proper ATC frequency try your new approach ;).
Nikos. -
Anyway, correct me if I’m wrong, but shouldn’t you be flying at an altitude that clears all known nearby terrain anyway? What airbase was home plate?
Your next step in the case that you are offset of the defined track of the procedure is to climb as soon as possible to the minimun safe/sector altitude, as defined in the chart.
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The barometric altimeter is the reference instrument. If you were found to be using the radio altimeter instead of baro as primary reference that would get your fired from any airline or air force. Upon returning home you get QNH (or QFE if your air force is still in the stone age) from the controller immediately before beginning the approach. You don’t assume it’s the same as when you took off a few hours ago.
The only time the radio alt is primary reference is for DH on Cat II/III approach which the F-16 is not rated for. There’s absolutely nothing wrong about crosschecking with it but for example if there is a mandatory crossing altitude and the baro and radio disagree, the baro is right.
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That made me a little curious Frederf, why not QFE?
I mean, 0 on baro altimeter should be runway right?No, I’m not educated on Q-codes
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@theOden:
That made me a little curious Frederf, why not QFE?
I mean, 0 on baro altimeter should be runway right?No, I’m not educated on Q-codes
Well you have right about QFE and barometric altimeter that will read 0 at runway.
Now why they (departments and ICAO and rest…) have chosen the usage of QNH instead of QFE, my guess is for traffic deconfliction at close airports.
Think 2 airports very close but with different RWY elevation. Then the aircrafts from them (approaching and departuring) that will be exactly at the same altitude, will gone have/say different numbers of altitude according to their ATC QFE, means chaos…
Nikos. -
That’s it, and also for security reason. Some accidents already append because a wrong QFE has been entered from a terrain A to a terrain B with two different elevation.
For example that one :
