Terrain warning in heavy rain

[khasabman]

I had a terrain warning recently whilst descending at least 2800 feet above very flat ground. The weather was extremely heavy rain. The rad alt caption appeared at 600 ft and started decreasing at a high rate.
It has been mentioned that heavy rain can affect the rad alts and will generate false warnings.
Does anyone have any knowledge/experience of this?

[jtr]

The Airbi that my mob has (330/340's) will occaisionally have radalt probs during operation in a wet environment, but are you inferring that the warning was due to density of the rain, or moisture screwing up the sensor?

[Bally Heck]

Have had GPWS flying through hail @ 3000ft over the sea. As if flying through a CB wasn't scary enough! :(

[QAVION]

Most Rad Alt antennae are susceptible to moisture and should be protected from such. The proper operation of an antenna is dependent on there being good electrical bonding between the antenna and the aircraft. Moisture can affect this bonding. A ring of aerodynamic sealant is put around the antenna after installation to prevent water ingress, but this may deteriorate with age.

When pilots log known false GPWS warnings, the engineer usually removes the appropriate antennae and inspects the antennae and fuselage for corrosion and other signs of water ingress. After the inspection, and assuming the corrosion is minimal, the antennae-fuselage contact surfaces are cleaned and treated with a cocktail of chemicals and the antenna is refitted. Aerodynamic sealant is then applied around the edges of the antenna/e. The sealant may later be painted with non-metallic paint (so as not to interfere with the radio signals)... Noting here that there have been been cases of painters not using the correct paint.

It may be possible that the transmitted RA microwave signals reflect off heavy rain just as weather radar would. However, I haven't, as yet, seen anything in writing from the manufacturer of these systems relating to this.

Rgds.
Q.

[mcdhu]

Airbus A320 Series FCOM Bulletin 36/2 dated Apr 99 entitled 'Radio Altimeter Anomalies During Adverse Weather Conditions' goes into this. Unfortunately, I don't have the technology or the know how to post an extract, but suffice it to say that you can expect returns to be generated due to reflection from 'hail clouds or heavy rain. If energy received by the radalt is powerful enough, it will be validated and a height lower than the distance to the ground will be measured and sent to system users.'
Perhaps somebody could post a fuller extract from CD ROM.
Hope this helps.
Cheers
mcdhu

[PAXboy]

In my work in telecommunciations I've known heavy rain blot out a microwave radio signal. It was only travelling about two miles, so the signal had more than enough pumch but the rain and hail blanked it for a few seconds at a time a the front went through.

[Dockjock]

Our radalt frequently dings 200-700' above the alert altitude we have set, even in clear air. What is the cause of this? Very frightening in cloud when the barometric alt still reads 500 to go til mins and the radalt is dinging. Of course a quick scan down shows the needle in the correct position, but the chime and light are already on.
A/C type C208B.

[Bogner]

Interesting post.

Hadn´t really heard of this before until last night on approach about 4000´above terrain in a very mountainous area that I´d never been to before. 767.

In the middle of the biggest cell I´d ever seen, couldn´t get off track due terrain so had to ride it out. Hail so loud that couldn´t hear a thing in the flight deck.

Just as we were approaching the hold we got a "whoop, whoop, Pull up", Rad alt was indicating 800´last time I registered it.
We had been struck by lightening just prior to this which may have been a factor, but the rad alt was indicating strangely before this anyway. I guess it was the density of the hail that caused it in our case.

Never want to be in that position again to find out though!
:ooh:

[av8boy]

We had been struck by lightening just prior to this which may have been a factor...
Put down 30 degrees of flap. Striker now listen to me.... lift your nose, straighten your wings. You're coming in too fast, watch your speed. You're coming in too hot. Ease up on the throttle. Watch for that crosswind. Aim for the numbers, you'll have to dip your left wing. You're drifting, keep your eyes on the far end of the runway. You're too low damnit! Watch your stall speed. Ease her down, down.

Dave:eek:

PS...I just want to tell you both good luck, we're all counting on you.