Radio altimeters


Hi All

I just watched the entire 2 hrs of the latest version ILS lesson by Jayne and Howard. Thank you to both. That was so helpful!

I have a question on radio altimeter and I know Howard very briefly touched upon it. But just to get my understanding right:

RAs are found only on bigger jets and not GA aircraft? I was practicing an ILS on the default Cessna CJ4 but could not find a RA reading.

How do you know if an aircraft is equipped with RA? Which types have a RA?

Are the altitude callouts like 50-40-30 only heard in aircraft with RA?


Mod: I couldnt find another topic on RAs. Please feel free to move it if there is one.

Hi Karthik,

In general, RA are indeed mostly found on bigger aircraft such as airliners. The altitude callouts are part of the CAWS (Cockpit Aural Warning System) which in turn is part of and linked to the GPWS (Ground Proximity Warning System). Here again you’ll find these on most modern airliners and will miss them on most GA-aircraft or jets.

Do note that the crew needs to set a ‘minimums’ decision height manually in order to have the ‘approaching minimums’ and then ‘minimums’ call-outs such as on the A320neo.

I think, but I’m not entirely sure on the following, that the RA are mandatory for aircraft that are rated with CATII ILS-equipment as they make use of the lower DH-minimums in bad weather. As most GA-planes are not equipped with such heavy ILS-equipment they won’t head into such low visibility weather and don’t require a mandatory RA either. Again though, I’m not entirely sure on this last bit but knowing pilots as I do, one will come around here and correct that if need be. :wink:

Happy flying!


Almost all business jets have radar altimeters as do many turboprops. Any aircraft equipped with a true EGPWS or TCAS will have at least one RA and usually two.


Every business jet I’ve ever flown has been equipped with a radar altimeter. Twin Cessnas I flew 20+ years ago that were certified in the 1970s had radar altimeters.

The CJ4 in MSFS has a radar altimeter if using the Working Title mod. Above the PFD is a button labelled PFD Menu. Press that and RA/BARO can be set in that menu. I am not sure if the default CJ4 has RA or not but if on a PC, download the free CJ4 Working Title mod from


Correct, when flying a CAT-I ILS you aren’t even allowed to use RA. You only set DH bug to DH for CAT II/III approaches. As the terrain in front of the runway further out is not prepared (can slope up or down), the DH call will not correspond to the altimeter DA/MDA. If you don’t have to option to set DA / MDA bug (barometric) instead of DH (RA), then turn it all the way down and make a manual callout.

As an example, imagine a 100 ft cliff in front of the runway. If you set DH bug to 200 ft RA on a CAT I ILS approach the “minimums” callout will be 100 ft too late. Therefore the DH bug can only be used on runways certified for CAT II operations as the area in front of the runway is specially prepared and then only on CAT II approaches, for a CAT I approach you still need to use a DA/MDA bug to get “minimums” call at the correct barometric altitude.

As mentioned before, the reactive part of (E)GPWS is solely relying on RA, the predictive part uses the GPS geometric altitude and a terrain database. TCAS uses radio altitude to inhibit increase descent RAs (A bit confusing maybe but RA here = Resolution Advisory), then a bit lower all descent RAs are cancelled, then even closer to the ground all RAs are inhibited.

There are aircraft without radio altimeter which have some kind of terrain and traffic alert systems based on GPS geometric altitude and pressure altitude instead. I don’t know if you can officially call those TCAS and (E)GPWS. In short, the only use cases for radio altimeter is terrain awareness (GPWS), TCAS inhibit functions and CAT II / III DH callouts. Its not further used in flight.

The height callouts on approach are a GPWS function and can be chosen from a menu of options. Even on the same aircraft type, callouts can vary between operators. Usually the full list to choose from is: 2500 ft (radio altimeter alive), 1000 ft, 500 ft, 200 ft and then 50, 40, 30, 20, 10.

1000 ft and 500 ft are quite useless callouts in my opinion. On instrument approaches you usually perform a call at 1000 ft and then 500 ft above runway elevation to confirm stabilized. These RA callouts always interfere with that as the area in front of the runway is almost never completely flat, if there is a hill in front of the runway it might occur early so you always have to respond with “disregard”.


With Reference to Radio Altimeters, there is current a possible “RISK” issue with their use near 5G cell phone towers, or even possible active 5 G phones in the plane.

Only recently being flagged as a concern, year after cell companies were given the OK on switching to 5G.
Amazing how well the FAA & FCC communicate & work together :-1:

Same here in Europe, we also received a notice from manufacturer.

TCAS is something entirely else, don‘t confuse it. There are often the sane abbreviations that mean different things. TA/RA mode on the TCAS means Traffic Alert / Resolution Advisory. RA here is the „climb! Climb now“ advisory that will keep you from a cozy date with the other plane.

I read that in Europe the 5G frequencies were higher than in the states so we wouldn‘t have that problem. Also in the states there were several 100 MHz between the frequency ranges and the whole topic was a media theater. We‘ll see where this will lead to, I just LMAO again ^^

As I understand from our manufacturer there have been cases of erratic radio altitude reported in Europe as well. They are investigating the cause but they suspect the 5G network. I haven’t seen anything weird so far.

No confusion. Radar altimeter data is a required input to a TCAS II, it is what causes the automatic suppression of all resolution advisories below 1000 feet AGL. If you pull the radar altimeter circuit breakers on the ground, the TCAS self test will “FAIL”.

In fact, probably 95 percent of TCAS test failures are caused by problems with external inputs rather than the TCAS processor itself. In addition to at least one radar altimeter input, a TCAS also requires heading (from IRS or AHRS), barometric altitude (from the air data computer) and two-way communication with at least one Mode S transponder.


aaah, okay… I wasn’t aware of that one but it totally makes sense. I have always assumed that the TCAS supression came with the landing configuration but of course if you were flying “clean” at say 800ft for some reason an RA to descend wouldn’t come in handy. Thanks for the heads up.

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We just received a service advisory from MHJ/Bombardier on Saturday stating that the Rockwell Collins ALT-55 or ALT-1000 radar altimeters found on all CRJ models have been tested and verified to be resistant to 5G interference. The service advisory includes an AMOC to the recent FAA airworthiness directive. The AMOC includes a long list of airports where CAT II approaches are once again allowed.

This situation should have been resolved long before the 5G rollout went live, but in the US, the FCC and FAA have been at loggerheads for years. There is no love lost between the two federal agencies. Previous to the 5G problem, major disputes between them have concerned marking and lighting of antenna structures, FCC type acceptance of personal electronic devices used aboard aircraft etc.

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“Climb, climb now” is a reversal, the usual RA is “climb, climb” or “climb, crossing, climb”, a reversal is when you previously received a descent RA but the other aircraft isn’t following (or can’t follow) his RA and is also descending.

TCAS RA inhibit usually comes in three phases, first the “increase descent” RA gets inhibited, a bit lower “descent” RAs get cancelled, then all RAs get cancelled (TA ONLY mode). Vice versa when climbing.


So I did find a RA set option under REFs and PFD menu in the default CJ4. But was not sure what to do with it. And I couldn’t find the RA reading either when flying under 2500ft.

Btw I’m on xbox.

Radio altimeter/radar altimeter — Airborne electronic devices capable of measuring the height of the aircraft above the terrain immediately below the aircraft. They operate in the 4.2–4.4 GHz band.


Frequency range 1 (< 6 GHz)

The maximum channel bandwidth defined for FR1 is 100 MHz, due to the scarcity of continuous spectrum in this crowded frequency range. The band most widely being used for 5G in this range is 3.3–4.2 GHz. The Korean carriers use the n78 band at 3.5 GHz.

Frequency range 2 (24–54 GHz)

The minimum channel bandwidth defined for FR2 is 50 MHz and the maximum is 400 MHz, with two-channel aggregation supported in 3GPP Release 15. The higher the frequency, the greater the ability to support high data-transfer speeds.*


5G 3.3 - 4.2 GHz (low # connections – short range 100 meters max)
RA 4.2 - 4.4 GHz
5G 24 - 54 GHz ( Typical Longer range 5G Towers)

Probably a bug then, the RA / BARO is for your minimum bug. It will either call “approaching minimums” and “minimums” based on the selected RA or BARO.

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CJ4 is only certified for CATI landings, so I don’t think we should be using RA anyway (isn’t this tied to CATII/III decision heights rather than CATI decision altitudes based on baro?) That’s why I’m always using the BARO setting and just insert whatever DA is in the approach chart (but I’ll always ignore RA and DH).

Also radar altimeters aren’t very accurate above 2500’ or so, and as such cannot be trusted. Ref. Radar altimeter - Wikipedia

Radar altimeters generally only give readings up to 2,500 feet (760 m) above ground level (AGL).

RA isn’t displayed above 2500 ft, neither can you “fly” using radio altimeter. The only purpose of radio altimeter is “minimum” callout at minima based on RA, GPWS reactive modes and TCAS inhibit. You could be right, maybe the CJ4 doesn’t have a RA, but then the RA / BARO option should not include RA. I have flown a lot of planes which weren’t CAT II/III equipped, still those had a radio altimeter for the above reasons and terrain awareness. I would be surprised if the CJ4 does not have a RA.

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I think it does have RA but my understanding is that it should not be used for CAT I. We should only be using BARO instead. WT’s documentation states that:

The CJ4 is certified for CAT I ILS or LPV/WAAS approaches with no less than 200’ AGL minimums. BARO minimums must be used for all CAT I, LPV and other approaches.

So even if the RA option is there, it should be ignored. At least that’s what I do.