Low altitude Radio altimeter b777 specific question
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Low altitude Radio altimeter b777 specific question
dear all this was clicked in cruise with a b777 2000' below me. at first I thought my RA was erroneously reading but everything happens for a reason. I was wondering why 1920' for an a/c 2000 ft below me.
As some of you maybe aware there is an A/c installation delay for b777 about 80', I was forced to co-relate the two. The AID is primarily to cater for a 0' reading at main gear touch down. 80 feet accounts for the length of cable and height of RA transceiver at the bottom of fuselage.
The AID is calculated and is Fixed based on FLARE/Touchfown ATTITUDE of a/c with L/G extended.
here is my doubt -
q1) is 1920 feet actually reflecting AID
2) if yes what about the l/g position. If theoretically I put my gear down will there be an adjustment? Present picture is clicked in cruise with gear up.
3) in ditching on a calm lake with l/g up (l/g up as per ditching proc) will the RA correctly give my height over water?
incase I have assumed wrongly please correct.
this is for academic only and very specific to the b777.
any help is appreciated
Thankyou
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The rad alt works by transmitting a continuous wave signal which sweeps its frequency up and down, usually within 100Mhz of the central frequency of 4700Mhz. The frequency difference between transmitted and returning signals is an indication of the time the signal has taken to get from the tx aerial back to the rx aerial. Left uncorrected, this time difference would indicate the height of the aerials above the ground so it is compensated by subtracting the equivalent of a time correction so that the indication shows the height of the main gear above the ground in the landing attitude. You are correct that the timing correction also has to take account of the time the wiggly amps spend in the wiring.
If you are in level flight and you are looking for the real height between the aerial installation in the belly of the aircraft and a reflection below you it won't give it, it will under read by the difference between the aerial installation in the approach attitude and the bottom of the gear when down. I don't know where the aerials are on the B777 but 80 ft sounds like a lot to me, unless they are forward on the fuselage like the B757, in which case I would expect a raw rad alt readout to indicate a negative figure when taxying. Remember also that the reflection will come from the top of the aircraft below you so you would expect to get an indication of 2000ft - (the height difference between the static probe and the top of a B777) - (the height difference between the rad alt tx/rx aerials and the bottom of the gear in the approach attitude) plus or minus any cumulative altimeter error on both a/c.
As to your questions..
1. I'm afraid I don't know what AID is, maybe someone else..
2. No, it won't change the reading
3. No
If you are in level flight and you are looking for the real height between the aerial installation in the belly of the aircraft and a reflection below you it won't give it, it will under read by the difference between the aerial installation in the approach attitude and the bottom of the gear when down. I don't know where the aerials are on the B777 but 80 ft sounds like a lot to me, unless they are forward on the fuselage like the B757, in which case I would expect a raw rad alt readout to indicate a negative figure when taxying. Remember also that the reflection will come from the top of the aircraft below you so you would expect to get an indication of 2000ft - (the height difference between the static probe and the top of a B777) - (the height difference between the rad alt tx/rx aerials and the bottom of the gear in the approach attitude) plus or minus any cumulative altimeter error on both a/c.
As to your questions..
1. I'm afraid I don't know what AID is, maybe someone else..
2. No, it won't change the reading
3. No
Why assume the lower aircraft was exactly at 19000 feet? Maybe he actually was at 19080 feet.
Even the tighter RVSM standard allows an "altimeter system error" of ± 80 feet, and an automatic altitude control system error of ± 65 feet. Toss in the difference between the static port "altitude" and the top of the tail or fuselage (reflective surface), and the lower plane might easily (and legally) been flying with its highest radio reflector at 19080 feet, 1920 feet below you.
Or perhaps both aircraft were within the margin of error in opposite senses: You were 40 feet low (but legal) and the other plane was 40 feet high (but legal).
Even the tighter RVSM standard allows an "altimeter system error" of ± 80 feet, and an automatic altitude control system error of ± 65 feet. Toss in the difference between the static port "altitude" and the top of the tail or fuselage (reflective surface), and the lower plane might easily (and legally) been flying with its highest radio reflector at 19080 feet, 1920 feet below you.
Or perhaps both aircraft were within the margin of error in opposite senses: You were 40 feet low (but legal) and the other plane was 40 feet high (but legal).
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# Alex - 80' mentioned in the AMM b777. For ARINC installations, standard AIDs are 20, 40, 57 and 80 ft depending on the size of the aircraft. The B-737, for example, uses a 57 foot AID. Thanx for your reply. With regards to your answer 3 whether in a calm lake/surface water will the radio altimeter give correct height over water, may I ask any reference to that for b777?
# pattern - appreciate your input. I am pretty sure both a/c altimeters read exactly 29000/31000 it was a company b777 and there was no TURB. I have never seen a continuous cruise flight on a 777 with primary altimeters reading even 10 feet different than what's set on MCP. My assumption has no technical backing but AID is always subtracted from the measured reading. So 1920 was too much of a co- incidence hence the doubt
I wouldn't rule out any other possibility as you mentioned but intention is to dig deeper until proven wrong if possible.
# pattern - appreciate your input. I am pretty sure both a/c altimeters read exactly 29000/31000 it was a company b777 and there was no TURB. I have never seen a continuous cruise flight on a 777 with primary altimeters reading even 10 feet different than what's set on MCP. My assumption has no technical backing but AID is always subtracted from the measured reading. So 1920 was too much of a co- incidence hence the doubt
I wouldn't rule out any other possibility as you mentioned but intention is to dig deeper until proven wrong if possible.
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The speed of the signal in the coaxial cable between the instrument and antenna is, depending on the type of cable used, 60-80% of the speed of the signal in air - a difference called the 'velocity factor.' In addition the signal has to travel the AID distance twice, outwards and inwards.
The 80ft AID means that the cable installed between the instruments and the antennas is always 80ft in length and never cut to length. The certification requirements for the cable would specify a velocity factor. The instrument therefore 'knows' to subtract (2 x known 80ft x known speed of light x known cable velocity factor) from the measured delay. I would guess the subtraction is done in terms of time as it would reduce software complexity and the number of code paths, especially if the same instrument could also output in meters.
So for an 80ft AID the actual displayed value would differ from the 'raw' value by 96-128ft, allowing for cable velocity factors of 60-80%.
Last edited by Lascaille; 28th Jul 2018 at 04:36.
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One key point not mentioned: A radio altimeter measures geometric height and an altimeter measures pressure altitude.
Now when you are at ISA those should go together. But as i understand it and very simply put: When the temperature is higher than standard then the air is less dense so you need to "stack" more of it to get to the according pressure. And when the air is cold you need to stack a lot less. Now this also applies to differences in altitudes.
If the air is warmer than ISA you will be further than 1000 ft apart flying at pressure altitudes that are 1000 ft apart.
Now if the air is colder than ISA which probably was the case with your example than you will be closer together.
Now when you are at ISA those should go together. But as i understand it and very simply put: When the temperature is higher than standard then the air is less dense so you need to "stack" more of it to get to the according pressure. And when the air is cold you need to stack a lot less. Now this also applies to differences in altitudes.
If the air is warmer than ISA you will be further than 1000 ft apart flying at pressure altitudes that are 1000 ft apart.
Now if the air is colder than ISA which probably was the case with your example than you will be closer together.
1. Which “bit” of the lower aircraft is the rad alt sensing?
2. As pattern as full as said you could be low by about 40 feet, the lower aircraft could be 40 feet high,,,yet both are within tolerance...
Frankly I wouldn't expect to see an integral 1000’ feet on the rad alt in these sort of circumstances...
IMHO chucking in cable velocities and temperature errors might well be technically valid but is is heading into angels in the head of a pin/number of teeth in a horses mouth territory....
2. As pattern as full as said you could be low by about 40 feet, the lower aircraft could be 40 feet high,,,yet both are within tolerance...
Frankly I wouldn't expect to see an integral 1000’ feet on the rad alt in these sort of circumstances...
IMHO chucking in cable velocities and temperature errors might well be technically valid but is is heading into angels in the head of a pin/number of teeth in a horses mouth territory....
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Altimeters are very precise that's why most of the time all three instruments read exactly the same.
So they are allowed to be within a range but it does not mean they are.
Cable velocities are irrelevant as the instrument is calibrated accurately for touchdown.
Difference from the standard ISA temperature will in my opinion be the biggest error in this scenario. Now i could look up the formulas i guess but i'm just gonna be lazy and use an online calc i found, so no guarantees for it to be correct. (http://newbyte.co.il/calculator.php)
At ISA-10C FL 310 equates to 29967 ft MSL and FL 290 equates to 28031 ft MSL.
Difference is 1936 ft.
So this sounds mostly like what op posted about.
Agg-Karan do you remember the ISA deviation that day?
Now it also measures low because the centers of the airplane are let's say roughly 1960 ft apart for ISA-5 and then you still need to subtract the upper part of the lower aircraft and the lower part (including gear) of the upper aircraft. Which could anywhere from 30 to 70 ft depending on the types etc.
Now on a ISA+15 day the planes would be 2110 ft apart and the radio altimeter would likely read about 2050 ft.
But still ISA deviations definitively plays a big part in what is displayed.
So they are allowed to be within a range but it does not mean they are.
Cable velocities are irrelevant as the instrument is calibrated accurately for touchdown.
Difference from the standard ISA temperature will in my opinion be the biggest error in this scenario. Now i could look up the formulas i guess but i'm just gonna be lazy and use an online calc i found, so no guarantees for it to be correct. (http://newbyte.co.il/calculator.php)
At ISA-10C FL 310 equates to 29967 ft MSL and FL 290 equates to 28031 ft MSL.
Difference is 1936 ft.
So this sounds mostly like what op posted about.
Agg-Karan do you remember the ISA deviation that day?
Now it also measures low because the centers of the airplane are let's say roughly 1960 ft apart for ISA-5 and then you still need to subtract the upper part of the lower aircraft and the lower part (including gear) of the upper aircraft. Which could anywhere from 30 to 70 ft depending on the types etc.
Now on a ISA+15 day the planes would be 2110 ft apart and the radio altimeter would likely read about 2050 ft.
But still ISA deviations definitively plays a big part in what is displayed.
A radio altimeter foot is dependent on the speed of light in the medium (very close to c in thin air). A foot on your altimeter depends on how close to ISA the bit of the atmosphere you’re in happens to be. Sometimes the two measures coincide; sometimes they don’t.
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The AID delay compensates for length of antenna cables, antenna position, gear geometry and (typical) flare angle to ensure that the RA reads zero at the moment of touchdown.
Look at any landing video and see how much lower the bottom of the gear is with respect to the lowest point of the fuselage (or engine cowls if closer).
The Rad Alt antennas have a conical beam (not a pencil beam), but the system will use a point on the ground which is closest to the aircraft for display purposes. This is why you still get a reasonable idea of height above the terrain when the aircraft is banking.... unless you decide to bank at 90 degrees... in which case the Rad Alt display will disappear. This conical beam means the RA may use the top of the fin of the other aircraft. This reduces your 80' difference somewhat, as would the relative position of the pitot static sensors with respect to the bottom of the wheels.
Baro vs radio altimetry
With regards to your answer 3 whether in a calm lake/surface water will the radio altimeter give correct height over water, may I ask any reference to that for b777?
The Rad Alt antennas have a conical beam (not a pencil beam), but the system will use a point on the ground which is closest to the aircraft for display purposes. This is why you still get a reasonable idea of height above the terrain when the aircraft is banking.... unless you decide to bank at 90 degrees... in which case the Rad Alt display will disappear. This conical beam means the RA may use the top of the fin of the other aircraft. This reduces your 80' difference somewhat, as would the relative position of the pitot static sensors with respect to the bottom of the wheels.
Baro vs radio altimetry
It's quite possible that the point of reflection for an airliner from above isn't the fuselage, which is nice and round so is not great at returning radio signals, but the tip of the tail fin. There may be a return from the wings and horizontal stabilisers, but they'll be later than the return from the tip of the fin, and are ignored.
EDIT - just seen similar point being made in a previous post!
Last edited by msbbarratt; 19th Aug 2018 at 14:08.
Following on from my last post, an aviation “foot” in terms of pressure altimetry only bears a passing resemblance to the 6076.12 feet in a nautical mile or the foot that a metrologist might derive from a physical standard.
The only constant is that properly calibrated altimeters with the appropriate sub-scale setting will read the same when aircraft are close to each other at the same geometric altitude.
The only constant is that properly calibrated altimeters with the appropriate sub-scale setting will read the same when aircraft are close to each other at the same geometric altitude.
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Assuming zero geometric error (e.g. Your Rad / Alt transmitter to the upper part of the "Target" aircraft below would be less than 2000 ft, any rounding down of the digital read-out (Rad / Alt digital read-outs are never rounded up), 2000 feet of Pressure Height delta converts to 1920 feet geometric difference on an ISA - 9.15 C day.
Such a non-standard atmosphere would be evident to you at F/L 310 and Mach 0.827 (as shown on your attached photo) as a SAT of -55.57 C and a TAT of -15.18 C.
Best Regards,
Old Smokey
Such a non-standard atmosphere would be evident to you at F/L 310 and Mach 0.827 (as shown on your attached photo) as a SAT of -55.57 C and a TAT of -15.18 C.
Best Regards,
Old Smokey