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DocLightning
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Altimeters And Rotation

Thu Sep 30, 2010 11:52 pm

So I wonder how an aircraft knows how high it is off the ground. More specifically, I wonder how it knows this at rotation.

OK, I get the concept of both pressure and radar altimeters. But at the moment of rotation, the nose is suddenly quite high off the ground and the tail is quite close to it. Even if the altimeter were located right over the MLG, the act of rotation would increase the apparent altitude by increasing the straight-line distance from the emitter/receiver to the ground. At that point, precise altitude information is important, if I am to understand, because it's important to confirm that you are climbing before retracting the gear.

For that matter, how does the altimeter not read an artificially high altitude when the aircraft banks in flight? Does the emitter somehow articulate so that it's always pointing straight down?

Does my question make any sense?
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
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Fly2HMO
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RE: Altimeters And Rotation

Fri Oct 01, 2010 12:34 am



Quoting DocLightning (Thread starter):
because it's important to confirm that you are climbing before retracting the gear.

Well, all you need to know you're climbing is look out the window, or if IFR, check the VSI.  cheeky 

Quoting DocLightning (Thread starter):
Does my question make any sense?

I think I see what you're getting at. In both the static altimeter and the radio altimeter I would tend to believe the difference produced while banking is insignificant.

[Edited 2010-09-30 17:35:14]
 
ANITIX87
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RE: Altimeters And Rotation

Fri Oct 01, 2010 12:45 am

An aircraft (airline) has two altimeters.

1) Pressure altimeter - used in cruise and for height above mean sea level (MSL). This number varies based on the barometric pressure setting the pilot chooses, and, therefore, the correct pressure value for the environment is given to him/her by ATIS, to ensure accuracy. This is what tells pilot the altitude they are at, and is the reason there is no difference when banking at cruise.

2) Radio altimeter - used when closer to the ground, and gives altitude above ground level (AGL). The radio altimeter uses a signal that bounces off the ground to give an altitude reading. This is generally only indicated to the pilot when the value is under 2500 feet, for ground clearance and information on landing, so the pilot has an idea or how close the aircraft is to the ground. IIRC, this altimeter is located just above the main landing gear, for accuracy when touching down. It is also the altimeter references for the GPWS.

Others here will be able to provide more information than I am, but that's it in a nutshell.

TIS
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etherealsky
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RE: Altimeters And Rotation

Fri Oct 01, 2010 12:47 am

Quoting DocLightning (Thread starter):
But at the moment of rotation, the nose is suddenly quite high off the ground and the tail is quite close to it. Even if the altimeter were located right over the MLG, the act of rotation would increase the apparent altitude by increasing the straight-line distance from the emitter/receiver to the ground. At that point, precise altitude information is important, if I am to understand, because it's important to confirm that you are climbing before retracting the gear.

Radar altimeters are typically not used on takeoff (aside from acting as a secondary reference). The main instruments we are concerned with on takeoff are the airspeed indicator (for obvious reasons  ) and vertical speed indicator. The latter is used (along with visual confirmation by looking out the window!) to confirm a positive rate of climb.

In case anybody reading this isn't familiar with a VSI, it operates on the principle of a "calibrated leak" where the needle (well, on a standard steam gauge that is..) is deflected whenever there is a differential pressure across a diaphragm. One side senses current ambient pressure, the other is vented to atmospheric via a calibrated opening which restricts the flow of air in order to prevent instant equalization with the "new" ambient pressure of the "new" altitude. (Hopefully that made sense?)

Here's a link to a good explanation with a diagram.


Quoting DocLightning (Thread starter):
For that matter, how does the altimeter not read an artificially high altitude when the aircraft banks in flight? Does the emitter somehow articulate so that it's always pointing straight down?

I'm assuming you're referring to a radar altimeter? I've never used one myself but I'm fairly certain that it's only really used once you're established on final approach. The main altitude instrument will always be your pressure-driven altimeter.
"And that's why you always leave a note..."
 
KELPkid
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RE: Altimeters And Rotation

Fri Oct 01, 2010 12:52 am

I don't think an analog altimeter is sensitive enough to detect much of a difference   The display certainly doesn't have the precision to show it on an analog altimeter. Digital may be another beast entirely...

There is another component here, too: most airliners have ground sense logic built into them (i.e. it "knows" when the plane is on the ground!).

Quoting Fly2HMO (Reply 1):
Well, all you need to know you're climbing is look out the window, or if IFR, check the VSI.

Nah, can't have your head out the window in the airport environment. Gotta concentrate on those pesky instruments and ignore the other aircraft that you might hit  
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
411A
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RE: Altimeters And Rotation

Fri Oct 01, 2010 1:55 am

Quoting etherealsky (Reply 3):
The main instruments we are concerned with on takeoff are the airspeed indicator (for obvious reasons ) and vertical speed indicator. The latter is used (along with visual confirmation by looking out the window!) to confirm a positive rate of climb.

Wrong.
In every airline aircraft I have flown, the AFM (and company procedures) clearly stated...
The PRESSUE ALTIMETER is used for confirmation of a sustained positive climb , prior to landing gear retraction.

This was true for the early jet transports as well, using the first true air data system...KIFIS.
KIFIS...Kollsman Intregrated Flight Instrument System.
Forget the VSI....many times, duff gen.

Looking out the window?
Only for the light aircraft types.

[Edited 2010-09-30 19:01:21]
 
Okie
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RE: Altimeters And Rotation

Fri Oct 01, 2010 2:26 am

Quoting DocLightning (Thread starter):
For that matter, how does the altimeter not read an artificially high altitude when the aircraft banks in flight? Does the emitter somehow articulate so that it's always pointing straight down?


Doc one would assume that roll and pitch angle would have to be input to the radio altimeter or have its own internal gyro for it to calculate the correct distance. Most Radio Altimeters seem to be limited to 2,500ft agl. I would not be sure with a 30deg bank at 2,500ft agl if much of the radio beam would even be reflected back at that angle of incidence to a flat terra firma.

Okie
 
Mir
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RE: Altimeters And Rotation

Fri Oct 01, 2010 2:53 am

Quoting DocLightning (Thread starter):
For that matter, how does the altimeter not read an artificially high altitude when the aircraft banks in flight? Does the emitter somehow articulate so that it's always pointing straight down?

The radar altimeter is really only used close to the ground, where you wouldn't want to be in a bank. So even if there were some error created by banking the airplane, it wouldn't be much of a factor.

-Mir
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rwessel
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RE: Altimeters And Rotation

Fri Oct 01, 2010 4:24 am

I think everyone is seriously overestimating the directionality of the radio-altimeter beam.

Since this is for short range only (hence no power restrictions), resolution and discrimination are non-issues (you only need to resolve a *planet* from half a mile away), and you want the actual distance to the ground regardless of bank and pitch angle, you'd really want to design the antenna with a fairly wide beam emitter. So long as the direct path to the ground is within the beam, the leading edge of the return pulse will be the correct distance. In fact an omni-directional emitter with a smidge of shielding (to block the path to any of the dangly bits of the aircraft) and a filter to remove all the returns less than a few nanoseconds (which would be the bounce off the shield) would work fine.
 
atlamt
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RE: Altimeters And Rotation

Fri Oct 01, 2010 12:29 pm

I don't recall seeing it on other a/c. But on the 737-800 the radio altimeter show's -4 feet when the a/c is on the ground. It was explained to us in class that in the landing flare the RA antenna is approx 4 feet higher and will read 0 when the mains touchdown.
Fwd to MEL and Placard
 
tdscanuck
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RE: Altimeters And Rotation

Sat Oct 02, 2010 12:09 am

Quoting atlamt (Reply 9):
I don't recall seeing it on other a/c. But on the 737-800 the radio altimeter show's -4 feet when the a/c is on the ground. It was explained to us in class that in the landing flare the RA antenna is approx 4 feet higher and will
read 0 when the mains touchdown.

It happens on other aircraft too. The RA's only real purpose in the world is accurate altitude above ground level for approach/landing, so it's calibrated for the distance from the lowest wheels to the ground when in typical landing attitude. In all other situations, it will be off by a few feet.

For everything else, you use the pressure altimeter which is completely insensitive to yaw, mostly insensitive to roll, and does show altitude changes with pitch commensurate with the location of the static ports relative to the CG.

Tom.
 
Pihero
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RE: Altimeters And Rotation

Sat Oct 02, 2010 1:12 pm

Quoting ANITIX87 (Reply 2):

1) Pressure altimeter - used in cruise and for height above mean sea level (MSL).

Wrong. The altimeter is one of the most used instruments and it's used AT ALL TIMES during the flight.To say "height above mean sea level" is a fallacy : we're talking about an altitude, referred to the local sea level pressure (and as a matter of fact it's not even measured, but calculated ).

As 411A said earlier in his 5 : "The PRESSUE ALTIMETER is used for confirmation of a sustained positive climb , prior to landing gear retraction."

The reason is that the IVSI has that "I" in its name , which means either "inertial" or "instantaneous" - vertical speed indicator.The idea is that one or two accelerators - just little weights-and-springs inside the pressure tubes : they would react quickly to a vertical acceleration, thus 'leading' the instrument indication. During takeoff, the classic call-out 'Positive rate of climb' which triggers the command "Gear up" has to be verified by the altimeter needle being "unstuck".

Quoting ANITIX87 (Reply 2):

2) Radio altimeter - used when closer to the ground, and gives altitude above ground level (AGL).

What you call "altitude above the ground" is simply called "height" (an alitude refers to areference pressure).
Contrarily to the common understanding, a radio altimeter works no longer on "radar pulses". Instead, the technology is about FM/CW or Frequency modulated continuous wave, which uses a totally different principle, hence the reason we now refers to "radio altitude" instead of "radar alt".
The beam is wide enough not to be influenced by the aircraft bank angle.
Apart from its use - mandatory in low vis procedures - close to the ground, the radio altimeter has the all-important function of providing the GPWS its radio-height information . Finally, the radio altimeter provides the auto-flight systems of various aircraft with height info required for mode switching or changes the systems need.The most obvious ones are the arming of the "FLARE" and ROLL OUT" modes of auto land...some other uses are less obvious (but this is another discussion...)

[Edited 2010-10-02 06:19:25]
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Okie
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RE: Altimeters And Rotation

Sat Oct 02, 2010 3:13 pm

Quoting Pihero (Reply 11):
The beam is wide enough not to be influenced by the aircraft bank angle


  

Thanks Pihero, now it is starting to make more sense.

Quoting Pihero (Reply 11):
Apart from its use - mandatory in low vis procedures - close to the ground, the radio altimeter has the all-important function of providing the GPWS its radio-height information .



Here I go assuming again, If you have a rising or falling terrain on the approach does the GPWS automatically add/subtract the difference to give you the correct altitude call outs in reference to the runway elevation?

Okie
 
tdscanuck
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RE: Altimeters And Rotation

Sat Oct 02, 2010 4:53 pm

Quoting okie (Reply 12):
If you have a rising or falling terrain on the approach does the GPWS automatically add/subtract the difference to give you the correct altitude call outs in reference to the runway elevation?

Although I can't say that none of them do, I've never flown with one that does...when the RA goes through 500', you'll get the "500" callout. The minimums callout is often attached to pressure altitude, not RA, so that one will work as you'd expect (relative to true runway elevation).

Tom.
 
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glen
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RE: Altimeters And Rotation

Sat Oct 02, 2010 5:05 pm

Quoting okie (Reply 12):
If you have a rising or falling terrain on the approach does the GPWS automatically add/subtract the difference to give you the correct altitude call outs in reference to the runway elevation?

No, the GPWS just works with its absolute measured radio height. It has no information about your location or the surrounding terrain. Therefore it is quite limited when you are flying towards rising terrain and could warn you too late.
The basic warnings are:
– Mode 1 : Excessive rate of descent.
– Mode 2 : Excessive terrain closure rate.
– Mode 3 : Altitude loss after takeoff, or go-around.
– Mode 4 : Unsafe terrain clearance when not in landing configuration.
– Mode 5 : Too far below glideslope.

The EGPWS (Enhanced GPWS) overcomes these deficiencies by working not only with the radio height, but also with your altitude and most important with a terrain database. With this help it has the additional warnings:
TERRAIN AHEAD and
OBSTACLE AHEAD
"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
 
Pihero
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RE: Altimeters And Rotation

Sat Oct 02, 2010 6:51 pm

Quoting okie (Reply 12):
If you have a rising or falling terrain on the approach does the GPWS automatically add/subtract the difference to give you the correct altitude call outs in reference to the runway elevation?

No, it's a "raw" datum.

Quoting tdscanuck (Reply 13):
when the RA goes through 500', you'll get the "500" callout.

As someone has said before, the redio altimeter comes alive at a height of 2,500 ft over something big enough to give a return... As normally the approch path of a runway is fairly flat, it's a good time to again x-check one's altimeters for gross errors.The height call outs are generally SOPs but just about everyone seems to agree on the following : 1,000...500... 50, 40, 30, 20, 10...

Quoting tdscanuck (Reply 13):
The minimums callout is often attached to pressure altitude, not RA, so that one will work as you'd expect (relative to true runway elevation).

No.For catII and III approaches, the call outs are RA-based, along with the "a Hundred above " call.
For that, you've just needed to have filled the "DH" box of the MCDU.
Contrail designer
 
DashTrash
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RE: Altimeters And Rotation

Sun Oct 03, 2010 1:57 am

Quoting etherealsky (Reply 3):

Radar altimeters are typically not used on takeoff

We used them for 400, 1000, and 1500 ft callouts. All have some sort of action associated with them. Easier than deciphering it off the pressure altimeter and on the East Coast there generally isn't enough terrain to make much of a difference.

Quoting etherealsky (Reply 3):
'm assuming you're referring to a radar altimeter? I've never used one myself but I'm fairly certain that it's only really used once you're established on final approach.

See above, but definitely most useful on the approach.

Quoting tdscanuck (Reply 13):
Although I can't say that none of them do, I've never flown with one that does...when the RA goes through 500', you'll get the "500" callout. The minimums callout is often attached to pressure altitude, not RA, so that one will work as you'd expect (relative to true runway elevation).

EGPWS takes your GPS position into account for the 500 ft call.

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