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Radar Altimeter / RADALT

If you try using rad alt at places like ELLX you have a good chance of ending between the approach lights.

On an ILS?? I can’t see a use for RADAlt other than as a safety tool on an NPA.

EGTK Oxford
Using the radar altimeter for the final approach was one of the main factors of the TU-154 crash in Smolensk in 2010 killing most of the Polish government. The navigator should have used the baro altimeter after getting local QNH but instead switched to radalt for final and failed to realize that there is a valley in final before the airfield. So he called out too high altitudes to the pilot when in fact the aircraft was below runway altitude and the go around warning was too late in the end. Vic

Last Edited by vic at 17 Mar 01:36

I have always asked RADALT equipped GA pilots on exactly how they use it but never got a detailed answer.

Nor have I ever seen a detailed answer posted on a forum.

For the cost of one – best part of 10k – I would want to get something real out of it.

It’s an expensive way to implement a gear warning. It would be a lot cheaper to use GPS i.e. GPWS but again I am not aware that any GA aircraft’s GPWS links the gear into that. The “five hundred” warning (which I get even with my G496) would be an ideal point to do a gear warning if the gear is still up. Do any of the GNS GTN G500 G1000 systems implement a gear warning? Presumably you would need to have paid for the TAWS option, and the “certified” (not sure what that actually means) version of TAWS is quite pricey.

Last Edited by Peter at 17 Mar 10:00
Shoreham EGKA, United Kingdom

The TSO document specifiying what Class A and B TAWS must have and do is here.

The Meridian has TAWS-B as an option. Normal G1000 terrain is not a TSO’d TAWS system although obviously it does quite a lot of the same things.

EGTK Oxford

I can’t see a use for RADAlt other than as a safety tool on an NPA

Yes, but as a safety tool, it is already an integral part of GPWS. A radalt display or callout on an NPA is more confusing than anything else. When yours calls 1000ft, you will be more than 3NM (or over 5km!, that can be over another valley in the alpine region…) from your runway. The 1000ft spot elevation 3MN away is in no way related to your threshold elevation. When you hear your thing call “1000”, you can be 500ft above the runway or 1500. Which makes a big difference when it comes to configuring your aircraft for landing. Therefore, these “gate” altitudes, typically 1000ft and 500ft are never measured with the RA, but with the altimeter.

Nor have I ever seen a detailed answer posted on a forum.

So I give you a very simple one now, coming from the operating manual of the company I fly for (there’s one Cat II approved aircraft in the fleet for which it is different): “Radio altimeter is not to be used.” Which is very similar to what our operating manual and also Cessna writes about the AoA indication: “No to be used as primary flight instrument”. Our unofficial procedure is to always set the decision height to 200ft, which is the lowest we are allowed to descend to on the best approach. Simply as a wakeup call in case both pilots got distracted or set a wrong QNH. But one must be careful when doing so, because on some airports, high terrain in front of the runway may trigger that “minimums” call too early, resulting in an otherwise perfect approach ending with an unnecessary go-around. Because, as I wrote before, all “minimum” callouts have to be followed, wether they come from human or machine. Therefore in cases like that, the radio altimeter is best turnded off.

Additionally, our company got the high GPWS calls disabled (namely the 2500 and 1000ft calls, unfortunately the 500ft call can not be turned off in our installations) because these figures don’t fit our gates. And as we have a proper two-dimensional terrain display on the MFD which gives a lot more situational awareness than an arbitrary spot elevation, there is nothing lost in terms of safety.

EDDS - Stuttgart

Has anyone here installed a radar altimeter, recently?

I have known a few TB2x owners with one but all were done many years ago. And one of them, a GT, was burning out large numbers of KFC225 autopilot servos, which may or may not have been related. Everybody loved him because he got the Honeywell extended warranty to pay for it. But nobody I have known could explain how exactly they used the RADALT.

Shoreham EGKA, United Kingdom

JasonC wrote:

On an ILS?? I can’t see a use for RADAlt other than as a safety tool on an NPA.

Not for a CAT I ILS, no.

ESKC (Uppsala/Sundbro), Sweden

If it is only for a gear up alert, maybe this would be useful. It’s for non-certified aircraft and provides an audio height call-out but doesn’t (AFAIK) integrate with a gear up switch. However, it’s built around a Garmin LIDAR unit, which you can buy separately for $150. The Garmin unit has a PWM output whereby the pulse width varies with distance, so it should be straightforward to hook it up to a gear up switch and generate an alert.

EGMA / Fowlmere

AltRadar is only useful for autoland and flaring with proximity to ground on those aircrafts with high inertia (combined with AoA) other than that barometric and gps altitudes should be enough?

I will not rely on it to avoid terrain or anything else, a bit like using car parking sonar for a self driving car along the country side…

Last Edited by Ibra at 11 Jul 20:52
ESSEX, United Kingdom

Its used on ils cat 2&3 for DH, rather than baro which is used for cat 1. So not a lot of use unless you are cat 2/3, apart from maybe extra awareness,… but gps does that now

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