50...40...30...20...RETARD...RETARD??? Radioaltimeter explained by "Captain"Joe - YouTube

Dear friends and followers, today I'll be answering another great question many of you have asked me the past

Why do you hear this voice?

50 40 30

shortly prior [to] landing in many approach and landing videos you see here on YouTube, so let's get started!

[Intro]

The voice you can hear in the background is the height readout of the radio altimeter.

The concept of the radioaltimeter is fairly easy.

You have one transmitting antenna and one receiving antenna installed at the rear part of the fuselage.

The transmitting antenna emits radio waves leaving the aircraft

downwards until hitting the Earth's surface and

most of the radio waves get reflected and travel back up to the plane where they get picked up by the receiving antenna.

The radioaltimeter computer then measures the time it took for the radio waves leaving the aircraft,

getting reflected by the Earth's surface and received by the receiver antenna.

Now, radio waves travel at the speed of light,

so the speed of light divided by the time it took will give you a

distance or in our case, a height.

The calculation looks very similar to this:

OK, now, let's look at the speed of light, which is normally given in meters per second

OK, this means that it travels at 299,792,458 meters per second.

Now, the thing is in aviation that most of it isn't given in feet so but that is a very simple calculation

Just have one meter equals

three point three

feet

Meaning you just have to multiply three by three times this will give you the speed of light in feet per second.

Okay, there we have it.

Now we have the speed of light in feet per second and now let's say that the radio altimeter has measured a time

from one radio wave transmitting

downwards, getting reflected and getting picked up again with the receiving antenna and the time it took to do that is

zero point zero zero zero zero zero

five zero five

seconds now as you may you can see this is the simple rule of three so what you're going to do is you just...

Can already see that you can cross out the seconds and you've got left over the feet.

So just multiply this times this divided by one gives you

Five thousand feet it's as simple as [that] so we've successfully

Calculated the five thousand feet but you have to divide that by two

Because you only want the distance down with and all the way back up again

So that will give [us] two thousand five hundred feet so on the Airbus A320 family the radio altimeter

becomes active at

2500 feet and announces that by a synthetic voice calling out

"Two thousand five hundred" so as a pilot, you are now aware the radio altimeter has come active and it's working normally

Side note if there were to be a problem with the synthetic voice computer

But the readout comes up on the pilot flight display, the pilot

monitoring would call out "radio altimeter live", following by a

confirming check by the Pilot flying and any further callouts would have to be made by the pilot monitoring

so when you make use of the radio altimeter

When it's [flying] down any kind of approach in bad visual weather conditions

It is a nice help showing you how high you are at the moment.

but you should be aware of your geographic surroundings because

your radio altimeter gives you a height above ground

indication and not a barometric altitude,

meaning the radio altimeter can show a lower height than the barometric altitude like in

this given picture. This can be hazardous and

Confusing if the terrain below is constantly changing so you want to concentrate on the barometric

altitudes rather than on the radio altimeter readout.

So where is it useful? It is a great help during landing and flare

when coming in for landing, your eyes start transitioning from the instrument onto the runway

so that you can adjust your lateral axis of the plane with the runway centreline and

as you are in a constant descent, the radio altimeter will read out their height above ground.

Now, when it calls out "50"

you will already be over runway tarmac,

making the readout very accurate and from that point on it's more or less a countdown

To [the] point where you reduce your sink rate or better known as the break and start your flare

The flare should be initiated at 20 feet on Gears 80/20,

and the famous call-out "RETARD, RETARD"

will remind you to retard your thrust levers to the idle power.

Haha, yes, very funny, the Airbus calls the pilot a retard. That joke is getting really old.

No matter if you're flying with the autothrust engaged or manually it will always remind you or in some cases

advise you to pull back the thrust levers. The Airbus A320 family is

equipped with two radio altimeters, one for the Captain's primary flight display and another one for the

Copilot side. In case that one radio altimeter

fails, both flight displays will indicate the height of the remaining one

but in that case the aircraft would be reduced in its landing capability down to Cat[egory] 2

Which would make an automatic landing of the aircraft not possible, [but] more about the auto-land and the ILS

categories in future video so [as] you can see it is a

Great help when it comes to landing and flaring the aircraft just before touchdown

and it's very reliable.

I haven't had a flight yet where the radio altimeter was inoperative or failed during approach.

By the way, the synthetic voice comes out of the loudspeaker

Fitted closely to the primary flight display.

And it overrides the volume control even if the speaker is turned off.

I hope you enjoyed the short little video today about the radio altimeter

Make sure to subscribe my channel for more detailed videos in the future and spread the word.

Thank you much for your time. Wishing you all the best your Captain Joe

You might ask why are the antennas placed at the far rear of the aircraft and not below the cockpit where you as a pilot would be sitting?

Now, you've all seen planes land.

Now just prior [to] touchdown the cockpit is higher in the air than the tail,

due to the pitch attitude of the plane.

Now, looking at this picture a radio altimeter

measuring the height at the front of the aircraft would lead to a higher indication,

although the tail is a couple of feet lower.

Now, if you would draw a line from the landing gear to the radio altimeter antenna,

You would see it is fairly parallel to the ground so you could say that the radio altimeter

gives the pilots the information on how high

the landing gear is above ground in that crucial moment of the flare.