1. Junior Member
Join Date
Mar 2005
Posts
89

Would someone explain to me what this instrument is and how to use it.
Thanks

2. Member
Join Date
Mar 2005
Location
Ede, The Netherlands.
Posts
267

I usually fly the R4D by Rambow, Chaffin and Visser, which has such a radar altimeter. It's pretty simple: in contrast with the standard altimeter which is zero'ed at a certain level or barometric pressure, the radar altimeter measures the concrete distance between itself and the ground surface you happen to fly above at any given moment. So if you're flying at an altitude of 3275 feet above sea level over a mountain plateau that is 2900 feet high, your radar altimeter will tell you that you're 375 feet above ground surface.

I find it a very useful little gadget.

Jaap Verduijn.

A good explanation. I might add a caveat, though. A radar altimeter does NOT read the ground in front of you, so you can't use it to predict ground clearance ahead. It only reads what is directly below you at this instant. Certainly the military has terrain avoidance devices, including radar that looks ahead, but this is not the common radar altimeter.

Larry N.

https://www.flightsim.com/images/noimage.png

4. Senior Member
Join Date
Mar 2005
Posts
1,480

A radar or radio altimeter (different names for same gauge) simply put, sends out radio waves and measures how long it takes for them to return to a receptor. Because the waves travel at roughly the same speed in all conditions, it is simple to extrapolate the distance the waves traveled by measuring the time it takes for them to return to the aircraft. That distance is divided by 2 (since the wave traveled to the ground and back) and is displayed on some kind of gauge. Most radio altimeters only function below 2500' AGL, at which point normal pressure altimeters are sufficient...usually.

Remember, a pressure altimeter when set with the correct local barometric pressure, will only display your altitude above sea level. The radio/radar altimeter will give your true altitude above the terrain, regardless of whether it is the actual sea, a mountain top, or the roof of a building. Because of the cost, weight, and size of the radar, most GA planes do not have this feature.

To read it is simple. If you are above 2500' from the ground, it will not register anything or it will say "Above 2500' AGL." If below 2500' (not all go up to 2500' and some can go above that value), whatever value is displayed is how high above the terrain you are. If your pressure altimeter is steady and the radio altimeter value is reducing, then the ground is sloping upwards to meet you. Now would be a good time to pull up.

-----
Ken G:-wave

5. Member
Join Date
Mar 2005
Location
Elizabeth City, NC, USA.
Posts
216

The way it was explained to me years ago is that a Radio Altimeter works very much like a depth sounder in a boat. It lets you know how much room is below your aircraft. Most of the time it is set to 200-500ft and used as a reminder to check and see if the gear is down and locked before landing. Or if on instruments set it to the decision height and if it goes off and you can't see the runway then you do a go around. I also use it to do my power reduction after takeoff, after passing 400-500 AGL (above ground level-which is what this guage shows) I pull the props and power back to the climb setting. When flying low over water it is the most accurate altitude indication you have so we use it a lot when on patrol with the Coast Guard as a set limit not to go below for safety. Hope this will help you decide how you will use this guage.
Ken

3 Things in Aviation you can't use: Runway behind you, Altitude above you, and fuel you left in the truck!
The Difference between God & a Pilot: God doesn't think he is a Pilot!:-wave

6. Junior Member
Join Date
Mar 2005
Posts
89

Thats how I thought it worked but I was not sure. Thanks Fellows

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