Are radar altimeters reliable? Have a look at the below video, why does the 777's rad alt on the PFD read minus 6? Take a close look at the video and play it back, I note the PF called gear up at approx. 6 feet rad alt!

On liftoff when the rad alt. reads 0, at that very moment are the mains still planted on the ground? I ask because -6 to zero still represents 6 units [well feet in this case].

Are there any 777 drivers here who could shed some light on this?

How do you commercial drivers use your rad altimeter with regards to takeoff?

HERE IS THE VIDEO:

http://www.youtube.com/watch?v=goECZnQ5-Ag

Quoting Tarzanboy (Thread starter): why does the 777's rad alt on the PFD read minus 6?

The R/A is calibrated to read zero feet during flare on landing.

Quoting Tarzanboy (Thread starter): How do you commercial drivers use your rad altimeter with regards to takeoff?

You don't. The radar altimeter is pretty much meaningless during take-offs. It's essential during instrument approaches however.

[Edited 2010-07-20 15:10:11]

So do you rely on your altimeter then on takeoff? What if it is a low visibility takeoff?

Quoting Tarzanboy (Reply 3): So do you rely on your altimeter then on takeoff? What if it is a low visibility takeoff?

The only altitude readings you really use on takeoff is just to make sure you have a positive rate of climb. Once a positive rate of climb is established, you throw the gear up. The next time you use your altimeter is simply to comply with ATC altitude restrictions, which is a few thousand feet up.

Quoting Tarzanboy (Thread starter): How do you commercial drivers use your rad altimeter with regards to takeoff?

The only time I include it is if it is a departure where I have to turn before 400 agl. I wait until at least 100 feet to avoid dragging a wing.

Quoting Tarzanboy (Reply 3): So do you rely on your altimeter then on takeoff? What if it is a low visibility takeoff?

We really don't pay much attention to our altimeter during take-off/initial climb out as long as we're climbing. That's the whole point of taking off.

Some airports may have altitude restrictions or special terrain considerations, and some airlines may have set procedures at x altitude, but as long as the altimeter shows a climb and so does the vertical speed indicator then all is good.

[Edited 2010-07-20 15:54:35]

Radar altimeters are pretty accurate. They also tend not to be near the main landing gear (usually they're mounted nearer the nose), which means the deck angle of the aircraft changes the distance from the radar transmitter to the ground, and hence the measured altitude. As N707PA said, they're usually calibrated so that they go to zero as the mains touch in the normal landing attitude.

So when the aircraft has the nose on the ground (with a zero deck angle), the radar altimeter has a negative reading, and if it has a higher deck angle with the mains still on the ground (as happens during rotation), it'll show a positive reading.

Quoting Fly2HMO (Reply 2): It's essential during instrument approaches however.

Technically, it's only essential during CatII or CatIII approaches.

-Mir

I have heard radar altimeters also referred to as radio altimeters. What is the most common term?

We use it only briefly to engage prof and, if desired, the a/p (500') and to initiate a departure hdg. That's all..

Quoting Tarzanboy (Thread starter): why does the 777's rad alt on the PFD read minus 6?

The Rad-Alt is calibrated to read '0' with normal landing deck angle when the mains touch down. The antennas are located on the lower forward fuselage so this is why you see -6 on the Rad-Alt when the plane is level on the ground.

Cheers,

John

Quoting lowrider (Reply 5): I wait until at least 100 feet to avoid dragging a wing.

Cool problem to have.

Boeingfixer, exactamundo! Those antenna are really accurate and I get your drift now. So basically during the flare when the mains touch down the rad alt reads zero and as the nose wheels eats earth you will basically get the reading of the base of the fuselage [near to the antenna] to the earth and in the 777's case, it is about 6 feet, hence the -6 on the rad alt in the 777 cockpit! Good stuff, the proof lies in the below 777 walk around, the pilot can be used as an object to 'gauge' the 6 feet distance.

http://www.jpbellphotography.com/images/large/AA02_BW.jpg

airliners rules. Lots to learn!

[Edited 2010-07-22 16:28:57]

Quoting Viscount724 (Reply 9): I have heard radar altimeters also referred to as radio altimeters. What is the most common term?

radar is an acronym from the US navy for radio detection and rangeing, so a radar is using radio waves.

Quoting Tarzanboy (Reply 13): Boeingfixer, exactamundo! Those antenna are really accurate and I get your drift now. So basically during the flare when the mains touch down the rad alt reads zero and as the nose wheels eats earth you will basically get the reading of the base of the fuselage [near to the antenna] to the earth and in the 777's case, it is about 6 feet, hence the -6 on the rad alt in the 777 cockpit! Good stuff, the proof lies in the below 777 walk around, the pilot can be used as an object to 'gauge' the 6 feet distance.

do you mena that during the flare, the nose wheel is 6 feet above the mains, and so when the nose is down it is 6 feet under where it should be when it meant to be used.

Quoting Tarzanboy (Reply 13): Boeingfixer, exactamundo! Those antenna are really accurate and I get your drift now. So basically during the flare when the mains touch down the rad alt reads zero and as the nose wheels eats earth you will basically get the reading of the base of the fuselage [near to the antenna] to the earth and in the 777's case, it is about 6 feet, hence the -6 on the rad alt in the 777 cockpit! Good stuff, the proof lies in the below 777 walk around, the pilot can be used as an object to 'gauge' the 6 feet distance.

Not exactly, unless I'm misunderstanding you.

The radar altimeter antenna is affixed to the bottom of the fuselage, and when the airplane is sitting on the ground it will measure the distance between the fuselage and the ground, or approximately the height of the landing gear.

Now let's assume for a minute that the antenna was located directly between the mains. So on the ground, it would sense (let's say) 6ft. What's displayed in the cockpit will be *0ft*, because the radar altimeter is calibrated to subtract out that distance, because what the pilots care about is the distance between the wheels and the pavement. Actually that's a bit of an oversimplification - a radar altimeter mounted like that would probably read slightly negative because of the compression of the main struts with the aircraft fully resting on them.

In some ways, having the antenna mounted between the mains would be ideal, since you'd always have an accurate measure of the distance to the ground. Unfortunately, the mains themselves end up being visible to the radar, plus that a spot with a *lot* of stuff going on, and space is at a premium.

So the radar altimeter invariably* gets mounted much further forward, usually not too far behind the nose gear. Near landing and just after takeoff, the aircraft is at a significant angle to the ground. So the exact distance to the ground varies relative to the mains because the fuselage is a different angles to the ground at different time. The altimeter is calibrated so that it reads the correct distance when the aircraft is in the landing attitude. When level (or on the ground), the nose will be lower relative to the mains (then at landing), and the radar will read low. At takeoff the nose will be higher relative to the mains (than at landing), and thus will read high.

Hopefully some really bad artwork will help:





*There's no reason you couldn't mount it under the tail, but there are no real advantages to that, and you end up with a rather longer cable run.

This picture helps a lot, tanks

Didn't know it was really that complicated. I remember a question from ATPL that said that Radio Altimeter shows height from main wheels to the ground. It made sense to me, so I didn't bother but now I see it is more complicated

rwessel, thanks for taking the time to illustrate your reply. Where did you get the 12 feet calibration?

The calibration of the system depends on the aircraft it is installed in.

Each aircraft type has a different antenna location, length of antenna cables, and typical pitch angle at touchdown. All of those go into the "calibration" of the system. Most large commercial aircraft indicate a negative number when sitting on the ground. The radio altimeter is also essential in the operation of the autopilot during a coupled approach. As the aircraft comes down the glideslope the system must be de-sensitized or it will overcontrol the aircraft. The radio altimeter provides the input to allows that to be done.

I always use the radar altimeter for callouts, climb profile compliance, etc. In every airplane I've flown, bleeds are turned back on after takeoff at a certain height, flaps are retracted at a specific altitude and airspeed, and climb power is set at a specific altitude.

You could use the barometric altimeter, but that requires math. Reading it off the radar altimeter is good enough.