Radar Altimeter

[flyheli]

How accurate should a Radar Altimeter be above 500ft.... the scale goes up to 2500ft on mine but it get's significantly inaccurate above 500ft.... any experience or ideas??
The equipment is new and the antenna unobstructed.

Thanks!!

[Simon853]

I don't know anything about these, but one thing I've always wondered is whether they're gimbled to always point straight down, or whether the're simply fixed vertical in relation to normal cruise attitude. (In which case I guess any deviation from that attitude in either dimension would translate as an error angle, which would magnify the distance reading with altitude.)

Si

[Aesir]

I find the dual RADALT setup in our B212´s is pretty reliable up to the limit of the scale at 2500´ in that they do not fluctuate! or behave erratically however they are usually about +100-200´ different from the pressure altimeter.

As far as I know they are not gimballed and will be somewhat inaccurate due to slant range expecially since the txnm/rec antenna is mounted on the tailboom.

However when landing on glacier it will usually show 30´ RADALT! Definitely not to be trusted in whiteout conditions.

[212man]

What are you comparing it with to come up with the errors?????

[Aesir]

uhhmm the pressure altimeter flying over ocean! Or are you asking "Flyheli"?

[last third]

Fly Heli,

I offer the following:.

Our machine uses a Mil Spec Radalt and is available between 0-1650 ft AGL/AHO (Above ht of obstacles) and must comply with the following:

1. Flying S/L altitude retention should be +/- 10 ft for altitudes 0-1650 ft.

2. Now turn the machine at Rate 1 for 180 deg, Steady state altitude retention (b/w 0-1650 ft) should be +/- 10 ft with +/- 30 ft transients allowed.

3. Now turn machine at > 30 deg AOB but < 45 deg AOB, Steady state altitude retention should be +/- 30 ft with +/- 60 ft altitude transients allowed.

This check is done with the AP and trim systems on and SAS on as well. Without an AP or trim system don't know what the requirements would be, however the following is generally accepted.

Rough rule of thumb is 10% of the altitude is the allowable error tolerance eg if you are at 10 ft then a 1 ft error of measuring of the RAD ALT is within tolerance, if you are at 500 ft then an error of 50 ft is within tolerance. The issue is how do you exactly measure 10 ft or 500 ft as the BAR ALT is a waste of space and not accurate enough for the comparison check. Need to hover abeam an object of known (measured) HT and x check RADALT, or use on board radar to determine return (need radar on board to do this).

Have Fun.

LT



:ok:

[McGowan]

Don't konow a lot, but have had nothing but trouble with the radalts fitted to the 206L and 407 (KRA10A type)especially when the antenna is fitted in the passengers chin bubble.
Only time they gave any kind of accurate or believable information was with the antenna fitted under the belly of the helicopter. The engineers at Westpac in Newcastle made up a mounting box for the antenna and the other bits that placed them on the belly between the cross tubes. Did a good job too.
Have been trying for a number of months now to get the problem resolved, but it is slow going. Nobody can, or wants to have a shot at fixing it............

[Ascend Charlie]

Generally within 10%, and it will depend on what you are flying over - broken canopy gives variable readings of 50' or more which is your 10%. I really only need them when less than 1000' agl, and specially below 200'. Good for reminders about wheel extension and critical decision points.

They don't look forward at all, so they have no purpose as a forward-looking radar to warn of approaching terrain. They just look straight down.

[flyheli]

Thank you very much for your inputs so far!!!

I was flying along the shoreline and used the GPS and Altimeter to compare and at around 1000ft I had a reading of 700ft on the RA... definitely more than 10%... someting must be wrong then.....???

[John Eacott]

What result do you get when you use the "press to test" button on the RA? Depending on the model it would normally read 50' or 100' (IIRC), with a limited margin of error which would be found in the manufacturer's handbook.

If the PTT exceeds limits, that's a fair indication that the readout will be erroneous.

Then again, your pressure altimeter could be indicating up to 300' out, not the radalt.

[puntosaurus]

Re: Simon853's post, here is an extract from the Mull enquiry which describes what the Radalt is doing. It may be helpful in answering flyhelis query also.

The radar altimeter looks at the nearest return it gets from the terrain, so if you are flying along next to a cliff it should return a distance to the cliff rather than a distance to the ground beneath you. There is effectively a cone over which the radar altimeter works which is fixed relative to the aircraft, so the actual piece of terrain which the radar altimeter will indicate the distance from is dependent on the aircraft attitude and the profile of the terrain.

Re. Aesir's glacier point, wasn't there a crash in a whiteout in the last couple of years that was put down to the unreliability of radalt over ice/snow ?

[Arm out the window]

I must admit that I haven't studied the inner workings of radalts closely, but I think the idea that they read the 'nearest' terrain to you is erroneous - Puntosaurus, if I read that quote of yours correctly it's suggesting that if you're hovering 20 ft away from a cliff and 150 ft above the ground directly below you, it will read 20 ft?!
If that's the case, then they should read your correct height AGL with any angle of bank on, as it implies that the beam is omnidirectional. I have always believed that the beam is quite directional, ie out of the transmitting antenna (under the forward part of the tail boom, say) and bouncing back up to the receiving antenna to be processed, and that's why they break lock when you put a decent amount of bank on.

[delta3]

Hi

I have been using a TRA3500 on an R44, and would agree with last third's post.

The specs state of FreeFlight

0-100 ft : +-5 ft
100-500 ft : 5%, so 5-25ft
500-2500 ft : 7% so 30-175ft


Flying over the ocean quite a bit last month at different altitudes, I would tend to agree with these figures (most of the time errors are smaller, pe at 1500 ft : 25 ft).

If not over the ocean, then the cone effect indeed exists, but also the effect of delays in the readings, if flying at some speed.

D3

[puntosaurus]

My reading of the quote is that (a) the radalt sends out a cone (ie somewhere between pencil and omni, but nearer pencil), (b) the propagation of the cone is fixed in relation to the a/c attitude (ie. not gimballed), and (c) whatever results in the first reflection back to the a/c is what is reported as height. I agree that the expert witnesses example chosen and use of the word 'nearest' is not helpful in that context, but I think his general drift gives the correct picture.

[delta3]

Hi

By reading the postings, it feels like a 20°x2=40degrees upto 30°x2=60 degrees cone would be OK

It should pick up the minimum over a reasonable surface below.
I see no value in NOT detecting the cliff in the given example 20ft on 150 feet, I hope it does detect the cliff. At 150ft/150ft it should not detect the cliff.

The angle of the cone also solves the problem of being not guimbaled and still providing correct reading with banking up to 20-30 degrees.

One of the days I'll check this with the TRA3500

Added : by re-reading the TRA3500 specs I found : "Antenna's response angle when mounted +- 6 degrees from horizontal = Dual : +- 20degrees for pitch, +- 30 degrees for roll.

So the designer adapted the cone for 30 degrees banking and 20degrees pitching.

At 150 ft it should detect the cliff from 50 ft at the front/rear, and from 70 feet sideways.

D3

[Two's in]

As an example of the radalt's I have used , on an IMC flight across the English channel it would show 300 feet AGL/ASL all the way until you impacted the White Cliffs mid way up. It is a line of sight device and shows what is directly below the transmit/recieve heads. With any forward speed on, you should treat as an indicator of past events, not future ones!

TI

[puntosaurus]

Actually following the delta3 calcs, you'd get a warning about 1/2s before impact at 60kts, but I suspect that would be a little academic !

[delta3]

It refreshes at most 2 times per second, so you even don't get the 1/2 sec warning...

d3