RadAlt over forest

[Genghis the Engineer]

Has anybody experience of operating a GPWS equipped aircraft close to jungle/forest. I'm wondering whether the RadAlt tends to detect the forest canopy or the ground underneath? I've not operated in that environment myself, and the textbooks don't seem to say much.

G

[kenparry]

Although I can't answer that query, I have had false returns from a rad alt from very heavy rain. I had an occasion near Tangier back in the 80s, in a 737-200, when at around FL150 in torrential rain the rad alt was giving wildly fluctuating readings for several minutes. We had GPWS fitted but I can't remember whether it reacted. It does show that the rad alt will see things other than the surface.

[411A]

On the L1011, ground only, forest trees have little effect.
Lockheed...built to a slightly higher standard.

At Palmdale...USA.
With...proper Rollers, of course.

[Gin Jockey]

I don't know what rad alt the a320 has, but they must like forests if the french chainsaw massacre incident was anything to go by...

[Denti]

With the small amount of time we are close to treetops i cannot really say if the GPWS reacts to them, but would think there is at least some minor filterin in the RA circuit. However that does not prevent the nice and very unexpected "1000" callout during cruise when another one passes underneath you.

[RobinR200]

No, the used frequency is too high for trees to reflect properly.

[NSEU]

when at around FL150 in torrential rain the rad alt was giving wildly fluctuating readings for several minutes.

A common problem with RA's is the electrical bonding of the antennae. If the sealant around the antenna is not so good, water can get between the antennae mating surfaces and cause indication fluctuations. Your aircraft may not have been reacting to the rain below the aircraft.

RA's react to lakes, rivers, etc... Just how heavy was that rain?

Rgds.
NSEU

[ChristiaanJ]

I thought modern rad-alts worked on about 5000MHz (unlike the original AN-APN1 on 440 MHz ).
About the same as Wx radar. So they should 'see' real sh!t below, such as a wet jungle canopy.
You people got me interested.

[Lafyar Cokov]

Certainly for flying over the Jungle canopy at very low level (in a military helicopter), the radalt generally locks onto the tree tops but one does have the occasionaly spike down to the ground for no apparent reason.

[ChristiaanJ]

Originally Posted by Lafyar Cokov

Certainly for flying over the Jungle canopy at very low level (in a military helicopter), the radalt generally locks onto the tree tops but one does have the occasionaly spike down to the ground for no apparent reason.

Thanks, much as I expected... The beam from a modern rad alt is only about 20° or less, so any small clearing at the low level you're talking about would cause it to 'latch on' to the ground level momentarily.

[212man]

I agree - all the radalts in helicopters I've flown (including my current type which has EGPWS) see the canopy. It goes without saying that it depends on the canopy density - I'm talking about jungle.

I'm very surprised to hear of 1000 ft calls caused by overflying another aircraft - I'd have thought that a spike like that would be filtered out (again, is for the helicopters I've flown)

[Fullblast]

Are you all talkin'about radar-alt or radio-alt?

FB

[Flying Serpent]

Ghengis,
Would you be thinking of a BAe 146 by any chance?

FS

[ChristiaanJ]

Originally Posted by 212man

I'm very surprised to hear of 1000 ft calls caused by overflying another aircraft - I'd have thought that a spike like that would be filtered out (again, is for the helicopters I've flown)

I was interested because somebody asked the same kind of question on a very different forum.
But unless you deliberately turn off the rad-alt ... think of slowly overtaking another aircraft below you on the same airway, same track, with the usual 300m vertical separation.... the "1000ft" call suddenly made sense to me!
Anybody else?

Originally Posted by Fullblast

Are you all talkin' about radar-alt or radio-alt?

Same difference, really.
Just grabbed a couple of manuals and books off the shelf.
Both British and French manuals refer to "radio-altimeter" or "radio-altimètre".
Original American literature refers to it as a "radar altimeter".

Strictly speaking it's radar - it measures a range.
But it's a rather special kind: CW FM radar ('continous-wave' 'frequency modulated' if you want to get technical...).

So it's just a matter of terminology and usage. But we're definitely talking about the same thing.

[ATPMBA]

It may not pick up the trees. I am basing that on an old story that in Vietnam, F111's had ground following radar that would guide the F111 a few hundred feet off the ground in IFR. In some places the jungle growth was 200 feet high or even higher. Needless to say, a few crews did not return.

I have also heard of some recent helicopter stories where the RA was not giving back accurate data while flying over water.

[ChristiaanJ]

Originally Posted by ATPMBA

It may not pick up the trees. I am basing that on an old story that in Vietnam, F111's had ground following radar that would guide the F111 a few hundred feet off the ground in IFR. In some places the jungle growth was 200 feet high or even higher. Needless to say, a few crews did not return.

The ground following radar is forward-looking, not down. A large clearing would tell you to go down, and the 200ft jungle at the other end might get you, if the hillside rising to 1000ft at the other end didn't.
Terrain following is hairy at the best of times.

The consensus here seems to be that the rad-alt will usually latch on to a thick jungle canopy, prefereably wet, but otherwise there may be blips. And non-jungle forests tend to be variable (eh, 411A?) with probably pine forests not showing up at all.

I have also heard of some recent helicopter stories where the RA was not giving back accurate data while flying over water.

Can you elaborate? Unless the surface was glassy calm, I find this difficult to imagine, so, again, I'm interested. (Ancient radar engineer freak here.).

[Flying Serpent]

Very low over Sabah Recently..

[Re-entry]

However that does not prevent the nice and very unexpected "1000" callout during cruise when another one passes underneath you.

I have only ever heard the '2500' auto callout in this instance.

[GlueBall]

We always hear the voice generated "One Thousand" when opposite traffic passes dead center beneath us.

[mnttech]

GlueBall and Denti,

Would you be as kind as to share what type of aircraft you are flying when you get the 1000' call? I have never heard of this, and find it interesting to say the least.

Thanks!