Radar altimeter error

[CaptKornDog 281]

Hello all,

 

I don't delve into the deep workings of gauges too, too often.  However, I dusted off an old aircraft tonight and noticed that at ground level, the radar altimeter depicts 8-12 feet usually.  I'm fine with maybe -2 to 3 or 4 feet, I see this slight error in real aircraft often enough to live with it.  8-12 unfortunately messes with my "10" foot GPWS callout so I'm hoping to tweak this.

 

Is this a gauge problem or somehow tied to incorrect positioning on the model or something else?

 

Any insight would be appreciated.

 

Thanks!

[COBS 25]

That appears to be correct.

The Radar altimeter is reading from the runway to your instrument in the aircraft .

Hopefully your Landing gear legs are longer than 2-4 foot long  , also the instrument presumably is in the centre of the aircraft fuse.

If the fuselage is 6 ft in diameter , and the aircraft was resting on it's belly , then the radar altimeter would read 3ft.

 

On the runway in the F-111 RADALT reads 7ft.

If you were in a Jumbo (747) the RADALT  would probably read in the vicinity of 20 to 30 ft.

 

As for your GPWS , I do hope that the alarm goes off a lot higher than 10ft.

The GPWS should be programmed to switch off the alarm either when you are on the ground or at a RADALT of about 50ft .

A GPWS  warning alarm closer than 50ft from the ground is useless , as at that height it's all too late

to react.

 

Cheers

Karol

[ckyliu 1,749]

The Radar altimeter is reading from the runway to your instrument in the aircraft. Hopefully your Landing gear legs are longer than 2-4 foot long  , also the instrument presumably is in the centre of the aircraft fuse. If the fuselage is 6 ft in diameter , and the aircraft was resting on it's belly , then the radar altimeter would read 3ft.

Surely they're calibrated to account for this, so they read zero at MLW?

[mgh 89]

Surely they're calibrated to account for this, so they read zero at MLW?

They are in the real world:

 

A properly calibrated radio altimeter indicates zero feet at the moment the aircraft contacts ground. This requires taking into account the height of the antennas above ground (at touchdown), the length and propagation delays of the antenna cables, and the inherent delays in the RT. The sum of all these delay factors is termed the Aircraft Installation Delay (AID). The RT usually employs external strapping to select the correct AID for that particular aircraft installation, using specified antenna cable lengths. Strapping connections and antenna lengths must match the installation requirements of the aircraft, or the altitude will not accurately indicate zero at touchdown. After touchdown, the weight of the aircraft may cause the radio altimeter to read slightly below ground level. (The less common pulse-type systems usually have a zero foot calibration adjustment accessible through the case of the RT.)

 

http://www.pacificavionics.com/viewPage.php?pgID=84

 

 

 

[iwebber 207]

Surely they're calibrated to account for this, so they read zero at MLW?

They're normally calibrated to read zero when the wheels touch down in the flare.

 

The sensors are usually near the rear of the aircraft so as you pitch down (after touchdown) the RA thinks you get higher.

 

It's usual to read a few feet on the ground.

 

Hope this helps,

 

Ian

[COBS 25]

OK, this could represent a minor problem , if you calibrate your RADALT   by the use of a factor , 

such as  -8ft in your case , the  RADALT  becomes a custom RADALT  that is specific to that

single aircraft model , and it cannot be used in another flight simulator aircraft model without

the corrective factor being changed.

 

NOTE : if as you say it is messing with your GPWS  then you will need to look inside that instrument 

because it also contains the RADALT code and your factor ( -8ft ) will need to be applied there as well.

 

Before you  include your factor , check your aircraft  RADALT at a couple of airports while on the runway,

allow the aircraft to come to a stop and settle , your RADALT should always be the same reading ,

Then decide what you want  the subtraction factor to be , you might want something other than ( -8ft ) ,

that becomes your choice.

 

Cheers

Karol

[mgh 89]

Checking a few default FSX aircraft suggests that the radar altitude is given quite closely by the absolute value of the main wheel vertical position minus that wheel's static compression - both from the [contact_points] section of the aircraft.cfg file. For example, the B747 the radar height on the ground is 16.76 ft. The main wheel vertical position is -20.0 ft, and its static compression is 3.0 ft . This gives 20.0 - 3.0 = 17.0 ft.

[CaptKornDog 281]

That appears to be correct.

The Radar altimeter is reading from the runway to your instrument in the aircraft .

Hopefully your Landing gear legs are longer than 2-4 foot long , also the instrument presumably is in the centre of the aircraft fuse.

If the fuselage is 6 ft in diameter , and the aircraft was resting on it's belly , then the radar altimeter would read 3ft.

 

On the runway in the F-111 RADALT reads 7ft.

If you were in a Jumbo (747) the RADALT would probably read in the vicinity of 20 to 30 ft.

 

As for your GPWS , I do hope that the alarm goes off a lot higher than 10ft.

The GPWS should be programmed to switch off the alarm either when you are on the ground or at a RADALT of about 50ft .

A GPWS warning alarm closer than 50ft from the ground is useless , as at that height it's all too late

to react.

 

Cheers

Karol

A GPWS altitude call out less than 50 feet absolutely has its uses....(eg "50", "40", "30", "20", "10"). In this case, each altitude is called out 10 feet too low since the system thinks it's higher than it really is.

 

(I also would love to see a 747 AMM that allows 20-30 error! They are certainly calibrated for fuselage, installation, and weight. Most RADALT systems I work on allow less than 5).

 

 

Now the question is how can I adjust this since I'm not the original author of the gauge/panel? Again, I don't really code gauges in FS.

 

Thanks.

 

Checking a few default FSX aircraft suggests that the radar altitude is given quite closely by the absolute value of the main wheel vertical position minus that wheel's static compression - both from the [contact_points] section of the aircraft.cfg file.

 

For example, the B747 the radar height on the ground is 16.76 ft. The main wheel vertical position is -20.0 ft, and its static compression is 3.0 ft . This gives 20.0 - 3.0 = 17.0 ft.

Interesting thought there.

[CaptKornDog 281]

(apologies if this double posted)

 

Changing the contact points and compression does indeed influence it.  However, it would sacrifice the visual contact points on the model (which correctly line up visually with the tires currently).

[edetroit 933]

Eh?...changing contact points?.....no no.

 

Rad alts are aircraft specific, there is no one size fits all.

 

Rad alt callouts are the greatest aid to a smooth landing ...a visual rad alt only good to the PNF IMHO

 

I use FS9. If I merge a panel...let's say the PSS 777 panel with the POSKY 777 and the rad alt is now incorrect, I rip out the PSS rad alt callout outs (silence them..not concerned about the visual readout) and add another gauge.

 

Rob Barendregt wrote a GPWS which one can calibrate for any aircraft for FS9 (maybe he did for FSX as well)

 

Stick that in your panel, put your aircraft on the runway at it's landing weight....tune to your liking and job done.

[CaptKornDog 281]

Eh?...changing contact points?.....no no.

 

Rad alts are aircraft specific, there is no one size fits all.

 

Rad alt callouts are the greatest aid to a smooth landing ...a visual rad alt only good to the PNF IMHO

 

I use FS9. If I merge a panel...let's say the PSS 777 panel with the POSKY 777 and the rad alt is now incorrect, I rip out the PSS rad alt callout outs (silence them..not concerned about the visual readout) and add another gauge.

 

Rob Barendregt wrote a GPWS which one can calibrate for any aircraft for FS9 (maybe he did for FSX as well)

 

Stick that in your panel, put your aircraft on the runway at it's landing weight....tune to your liking and job done.

Continuing on that example then, how do you remove (silence) callouts that are embedded into the RADALT/GPWS when the panel is programmed through a single .gau file? For instance, PSS like you mentioned...panel is more or less one single gauge without a separate, explicit GPWS entry I could simply delete from the panel.cfg.

[phjvh 0]

May be it is possible only to rename the original .wav files and keep the look of the GPWS.

Then use Rob B's, simply the best!

[edetroit 933]

May be it is possible only to rename the original .wav files 

 

Well I would use a audio editor and reduce the volume levels to zero.

 

edit* Mr Weber, If the sound files are in the main gauge file and not in a separate sound folder you have a problem.

[mgh 89]

If you start your flight on the ground then the correction is given by the difference between the PLANE ALT ABOVE GROUND and GROUND ALTITUDE variables That correction can then be saved for use later in the flight.

 

Incidentally, the PLANE ALT ABOVE GROUND and the RADIO HEIGHT variables seem to give the same values.

[CaptKornDog 281]

Well I would use a audio editor and reduce the volume levels to zero.

 

edit* Mr Weber, If the sound files are in the main gauge file and not in a separate sound folder you have a problem.

Yep, sounds like I'm stuck. That's too bad.