B777 Radio Alt Minus value

[supernova.surfer]

Hello All
Does anyone know why the Radio Alt PFD window shows a value of -2
while on ground ? what caliberation is that exactly , thanks in advance

Dee

[B-HKD]

Length of the 'truck'. Thus shows -2 when all wheels are down. On the 744 it is -8 with all wheels down.

[kijangnim]

Greetings,
Nose wheel in contact with ground?

[trimotor]

0ft would be with the main gear trucks tilted at the momnt of the aft wheels touching (or leaving the ground), so shows -2ft with the trucks level.

[FC747]

does anyone know what the RA reads on ground for the 737 - xxx?

[NSEU]

I suspect the negative value on the 777 is a combination of the following:

Strut compression
Bogey tilt
Location of Radio altimeter antennae on the fuselage
Aircraft weight/CG

The system will be calibrated so that in a normal flare, gear bogey tilted and strut extended, the radio altimeter will read zero at the moment the first wheels touch down.

does anyone know what the RA reads on ground for the 737 - xxx?

If you look at the photos on airliners.net, you can see some typical values... e.g. -4'

Rgds.
NSEU

[bugged on the right]

Flat tyres?

[747Flyer]

737 is -4' as I recall...

[Denti]

Either -2ft or -4ft, depending, can change around between those two.

[Fright Level]

You're in Amsterdam ???

v v v v v v v v v v v v v v




















(No need to point out the deliberate error, it was a joke!)

[Flaperon777]

The RA is caliberated (both on the 777 AND the 737NG) to show height of the underside of the fueselage versus the ground. Thus to counter the height of the landing gear the RA is 'platformed' at the approx landing gear height. Thus the fluctuation sometimes between -2 and -4 feet. Also worth noting that the RA transmitter and reciever are both located on the underside of the belly of the 777 (AND the NGs). Just behind where the pilots sit.
Hope that helped..

[supernova.surfer]

RA

Thanks for your replies

[Fantomen]

On the 744 it reads -8. The radar altimeter sensors are well forward of the main wheels so it makes sense that they would read zero when wheels first touch.

[NSEU]

Here's a diagram I prepared earlier...

[flighttest-engineer]

it is -6ft on the B757, if I remember correctly.

[M2dude]

Irrespective of type, all aircraft Rad Alts are calibrated to read MAIN WHEEL height above ground at approach attitude. Concorde read about -15' on the ground.