Radio Altimeters

[leewan]

Radio altimeters work on the principle of bouncing radio waves off the ground to get the actual height above ground.
So say, there's a huge building that's a few hundred metres wide and say 50 metres high( like a hangar or a factory) and under the approach path of the a/c below 2500 feet, would the RA reading be affected ? I don't think it does as i've seen a/c approach paths over such buildings, but why ?:confused:

[HR GREENIE]

You bet ,the Rad Alt will be affected be buildings as the radiated signal from the Rad Alt antennas will bounce off the roofs just as well as the earth .

Some Autopilots only use the rad alt for the last couple of hundered feet as a hight above runway for auto level out.
The actual glide path used by the autopilot is sensed via the Airports ILS system that coprises of a localiser (left/right deviation from runway center line) and glide slope . These are fed In to the Autopilot along with other Input to allow the Aircraft to fly a predetermind path to the end of the runway.
As the pilot flew over a building the Rad Alt Indicator would show his hight above the roof but would not effect the Autopilot .Ground Prox Warning may be affected ,Iwould have to read up on that one.

Hope this helps.

[leewan]

Would they not use the barometric altimeter as a backup instead ? And how about approach paths in urban areas with the ILS and glideslope inop ? Kaitak springs to mind.What would be the procedure like ? Wasn't the turkish 737 crash attributed to a faulty RA which caused the autthrottle to retard and crash ?

[HR GREENIE]

Yeh baro Is used as well as DME but If the ILS Is out then It would have to be a visual approach I guess or use an alternate Airport .

A lot of Airports are starting to use approaches aproved for GPS as the latest GPS are TSO rated for this . I.E they will provide vertical guidance due to the Increased accuracy of the GSP system They also run Baro Info to the GPS for redundancy If a satellite become unusable.

Can't answer the 737 issue I only deal with Helicopters S76,sorry.

May be part of the Ground Prox system ? as the Rad Alt would be part of this?

[leewan]

Thanks for the insight hr greenie

[NSEU]

Airports which are certified for autoland usually have a reasonable amount of flat ground before the runway threshold.

Baro is not used for autoland because it is not accurate/consistent enough. Rad Alt-based navigation doesn't rely on the pilot dialling in the correct barometric setting for the airport (QNH) every few minutes.

As for buildings, I'm sure the autopilot system is sophisticated enough to ignore momentary height variations. I know it will ignore the loss of ILS signals for a certain time period- it assumes that something (e.g. another aircraft) has briefly blocked the signals. There are a lot of intelligent algorithms like this built in to autopilots these days.

Rgds.
NSEU

[mtanz0]

The altimeter expects a gradual decrease in altitude when coming in for a landing. If the beam is suddenly bounced off the roof of a building and the return time is greatly reduced the system will know it's an erroneous reading and will probably resume reading once it gets a series of appropriately differentiated readings.

[leewan]

Thanks for the info guys.

[Fake Sealion]

Up to what sort of altitude can a rad alt be effective/accurate?

ie Can it be used at airliner cruising levels -FL30+

[leewan]

Nope, it's effective up to 2500 ft only.

[avionicsmoke]

Leewan,
Correct for Collins ALT55A and other makes but check the spec on the attached!


http://www.honeywellcommnav.com/documents/technical/52b_tech.pdf

[vickers vanguard]

avionicsmoke, nice find....I'm surprised by the 8000 ft claim.

[Bullethead]

G'day Fake Sealion,

Hi level radar altimeters are available, we had them in the C130Hs I flew in the Aussie Airforce. The NAVs used them for pressure pattern flying to work out absolute altitude. We used them up into the mid FL300s

Regards,
BH

[Fake Sealion]

G'day to you Bullethead!

Thanks for your reply. Thats interesting. I posted on the AirFrance thread on Monday requesting views about the viability of using Rad alt info or similar (GPS?) to provide the crew (and AP?) with another source of data other than pitot/static as back up, in case of failure of said pitot/static input. I did this after reading the report of a similar disaster (can't recall dates) following loss of pitot/static input. The crew had speed about right (from ground radar monitoring) but flew into the sea at night as they had no reliable height info and hence spatial awareness was comprimised.

For my trouble my post was deleted by mods and I was put in the sin bin for 3 days.:oh:

[Bullethead]

G'day FSL,

I think it would be easier these days to use GPS altitude info rather than hi-range radar altimeters and EGPWS should take care of hitting things when low level at night or in the murk.

The MODS have certainly been busy lately.

Regards,
BH.

[leewan]

Wow !:ooh: FL300. Why isn't this found on commercial a/c ? It could supplement the RVSM operations greatly.

[airflorida1]

Yes, your absolutely right, the RA will be affected