Setting QNH/Altimeter after GPS?
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Ask yourself why RVSM isn't predicated on GPS, but on QNE
True altitude isn't the same as indicated altitude; that much any student pilot knows.
In advanced systems, the air data computer considers temperature, temperature and airspeed and other factors when applying corrections to instrument indications, engine parameters, etc.
In advanced systems, the air data computer considers temperature, temperature and airspeed and other factors when applying corrections to instrument indications, engine parameters, etc.
I am not familiar with the true altitude terminology, btw. Is that a standard term in the States, or is it informal use to distinguish from barometric altitude?
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Because it's self-contained.
Do you mean that GPS is self-contained, and therefore unsuitable for RVSM? Do you mean that because GPS is suggested to be more accurate than a baro altimeter, that it's therefore unsuitable for reduced minimum separation airspace?
Which brand of barometric altimeter connects to the ADC?
I am not familiar with the true altitude terminology, btw. Is that a standard term in the States, or is it informal use to distinguish from barometric altitude?
True altitude is actual altitude. It is calibrated altitude, corrected for temperature.
Pressure altimeters have 3 basic classes of error (forget that PITCHBLOT nonsense - it doesn't explain things correctly!)
1. They're not perfect machines. But capsule hysteris, internal temperature effect and lag can be reduced by clever design, e.g. in servo-altimeters.
2. The connection between the altimeter and the outside atmosphere is not perfect. For example, aircraft static pressure errors and mach no. affect the actual static pressure. These can be corrected by the Air Data Computer and/or Pressure Error Correction Unit - a major need for RVSM flying.
3. The atmosphere in which the aircraft is flying is non-ISA. Or the local static pressure may have been affected by hill/mountain effects. These errors are non-aircraft specific and will affect every aircraft in the same vicinity equally. They cannot be universally corrected - because an identical atmospheric pressure compensation would be needed for every aircraft. Which is impossible to achieve.
GPS altitude may be useful for gross error checking, but can never be relied upon as a primary altitude reference.
No! The RVSM datum is 1013.25 mb (or hectopascals if you must) / 29.92 inches of mercury. This is often called 'Standard Pressure Setting' (SPS) - dear old Q-code junkies refer to it as QNE. Although I understand that the old morse query 'QNE' actually meant "What is your indicated touchdown elevation with 1013.25 as the pressure datum". The reply "QNE (n feet)" meant to the receiving aircraft that if it landed using 1013.25 it would see the (n feet) indicated aerodrome elevation at touchdown.
1. They're not perfect machines. But capsule hysteris, internal temperature effect and lag can be reduced by clever design, e.g. in servo-altimeters.
2. The connection between the altimeter and the outside atmosphere is not perfect. For example, aircraft static pressure errors and mach no. affect the actual static pressure. These can be corrected by the Air Data Computer and/or Pressure Error Correction Unit - a major need for RVSM flying.
3. The atmosphere in which the aircraft is flying is non-ISA. Or the local static pressure may have been affected by hill/mountain effects. These errors are non-aircraft specific and will affect every aircraft in the same vicinity equally. They cannot be universally corrected - because an identical atmospheric pressure compensation would be needed for every aircraft. Which is impossible to achieve.
GPS altitude may be useful for gross error checking, but can never be relied upon as a primary altitude reference.
Suppose that should read QNH? Mixing up QNH with QFE, perhaps?
Last edited by BEagle; 14th Sep 2010 at 08:06.
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Because what is self-contained?
You're not familiar with true altitude?
True altitude is actual altitude. It is calibrated altitude, corrected for temperature
Suppose that should read QNH?
No, QNE is correct--it refers to standard pressure, as in setting the altimeter to 1013.25 to fly flight levels.
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The incorrect confusion of QNE with the Standard Pressure Setting is, unfortunately, widespread
(and please let's not have another ******* boring thread about "correct" R/T
)
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Altimeters. They do not depend on bits outside the aircraft, same as INS.
Your answer is that it isn't used because it employs no external reference (you're not familiar with baro-aiding, I take it). If GPS is indeed more accurate and therefore the better altitude reference as some have asserted, then why is it not used for the primary altitude reference? What has external measurement got to do with the price of tea in china?
In fact many GPS units do consider barometric pressure as an input, but that's neither here nor there. Whether the GPS display considers barometric pressure or not, or uses any external input, shouldn't have any bearing on the reason that it is or isn't used as the primary altitude standard.
The fact is that despite most all aircraft in RVSM airspace, where separation standards are reduced and aircraft operate closer together in the IFR environment, are equipped with advanced GPS and FMS equipment, GPS is still not used for altitude reference.
We do use that, and other altitude data as an unofficial reference sometimes when giving metric altitudes in Chinese and Russian airspace; it's handy for giving a "passing through" altitude on our way to an altitude assigned in meters. Otherwise, GPS altitude remains little more than an interesting tidbit.
Suppose that should read QNH? Mixing up QNH with QFE, perhaps?
While 1013.25 might be a technical standard, it's not used for flight above the transition level. I set 29.92 or 1013 on the altimeter going up, and QNH on passing transition altitude on the way down.
QFE is an entirely different animal, used by Russians and parachutists.
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All aircraft at the same point in space will measure the same pressure (subject to minor calibration variances in an RVSM aircraft), hence this is a very robust system for separating aircraft.
Finally, the whole vertical separation infrastructure is based on pressure levels and ubiquitous and multiply redundant GPS is still not with us - so if you were going to define the equipment requirement for RVSM 20 years ago the only choice was barometric systems.
If in 20 years you were going to define a system from scratch to avoid impact with the ground and other aircraft, I would be very surprised if barometric altimetry (subject to human error and very large errors in absolute altitude) would be chosen over EGNOS/WAAS GPS (works without human interaction to an accuracy of a few meters)
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LH2's comment more fully was - Barometric pressure corrected for the local aircraft effects through the ADC (i.e. the 'consistent' pressure level) is used for RVSM because it is generated internally and has no requirement for reference outside the aircraft. In addition, the outside air pressure is something that can be measured quite accurately with multiply redundant pressure sensors and therefore has a high degree of resilience.
So far as accuracy, it's a subjective term. Barometric altimetery is accurate enough for RVSM, to be sure, but certainly not accurate in revealing true altitude. One's actual height is largely irrelevant when flying a barometric altitude or flight level, save for obstacles which couldn't give a whit about barometric altimetery.
As for resiliency, one may look to hysteresis for repeatability, but even in advanced systems, I see errors that are hardly resilient. I recently ferried several airplanes that had been sitting in the desert for a year or so, only to find that altimeters disagreed by several hundred feet at altitude. Many of the older Lears that I used to fly were ADC corrected on the captain side, but pure baro on the copilot side, and were nearly always at least 500' different in their displays. That the display might show this or that on any given day was always a matter of guesswork, as well as temperature and airspeed, for without correction and input, the baro altimeter was anything but resilient.
Improperly set, the baro altimeter is anything but forgiving.
Why then, is GPS not the golden standard for setting flight levels and maintaining them in the compacted, high speed world of the upper flight levels? If it's so much better at providing an altitude, as some suggested, why is it not used?
A barometric altimeter may freeze; static ports may seal off, and many utilize vibrators to ensure minimal hysterical errors (a Freudian discourse in it's own rite, I'm sure), yet they continue to work in the absence of generators or heat with as little effort as opening an alternate static port. The same can't be said of a GPS, which is always dependent on mother-battery, or father-generator for life.
Barometric altimeters have been used for many years and will continue to be used for many to come, for good reason, in the same plain, old fashioned way they've always been used. Why? They work.
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'Internal to the aircraft' was meant to be interpreted as not requiring systems remote from the aircraft. Obviously to measure static pressure you need a static port or three!
As regards the remainder of the post, thank you for so eloquently making the case for GPS altitude.
I suspect the only reasons GPS is not the standard are
As regards the remainder of the post, thank you for so eloquently making the case for GPS altitude.
I suspect the only reasons GPS is not the standard are
- it is only in the past few years without SA that the civil altitude accuracy was sufficient
- there is only one supplier of the system (the US military)
- there would be uproar if all aircraft globally were suddenly required to fit certified GPS and recertify/train aircrews controllers regulators et al (after all to separate the aircraft we all need to use the same measurement basis so everyone needs to use one system or the other)
After all, after extensive testing and trials, the UK CAA has only recently concluded GPS might have a comparable level of accuracy and resilience to the NDB
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As regards the remainder of the post, thank you for so eloquently making the case for GPS altitude.
Altimeters don't quit. GPS does.
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I didn't.
You pointed out many of the problems with altimeters.
GPS has different problems. On balance it may well be altimeters remain the more reliable of the two BUT there is a trend to greater reliance on electronics if only because such systems are ultimately cheaper. I am quite certain if a sufficiently reliable and compatible alternative could be found manufacturers would be delighted to ditch encoders and altimeters for a self contained matchbox backed up by another self contained match box that did the lot. A loss of all electrical power is becoming an issue whether it be in a Cirrus or an A380. In a DA40 or a DA42 in fact you will find the engines stop - that may not be a good idea but it is illustrative of the way designs are evolving at the lighter end of the market.
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I think everybody here really does know the real reasons why this and that is done a certain way.
Perhaps altimeters will be with us for ever, because they are simple and don't need a power supply.
But I can see one day everybody flying a precision altitude, GPS/EGNOS/WAAS derived. It would dispense with RVSM (which adds at least $100k - a totally mad and ludicrous amount for what you are actually getting for that money in equipment terms - to the cost of any plane capable of climbing up there, and a lot more than that to upmarket types) and would do away with QNH, QNH readbacks, the need to pay the salary of ATC staff reading out the QNH, etc.
One day, in CAS anyway, clearances will be delivered digitally and it would be silly to have a mechanical altimeter, with a servo motor driving the subscale knob, and the servo motor controlled by a computer which decodes the ATC message
Perhaps altimeters will be with us for ever, because they are simple and don't need a power supply.
But I can see one day everybody flying a precision altitude, GPS/EGNOS/WAAS derived. It would dispense with RVSM (which adds at least $100k - a totally mad and ludicrous amount for what you are actually getting for that money in equipment terms - to the cost of any plane capable of climbing up there, and a lot more than that to upmarket types) and would do away with QNH, QNH readbacks, the need to pay the salary of ATC staff reading out the QNH, etc.
One day, in CAS anyway, clearances will be delivered digitally and it would be silly to have a mechanical altimeter, with a servo motor driving the subscale knob, and the servo motor controlled by a computer which decodes the ATC message
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Torque Tonight wrote: "It's very not cool to set....."
This is an interesting thread, BUT -
will someone, preferably you, define for me what this is meant to mean?
Is it dangerous, illegal, foolish, likely to endanger oneself and/or others, or the practice wasn't promulgated when Miles Davis was playing?
The word "cool" used in aviation shpould in my book only used regarding temperature.....
This is an interesting thread, BUT -
will someone, preferably you, define for me what this is meant to mean?
Is it dangerous, illegal, foolish, likely to endanger oneself and/or others, or the practice wasn't promulgated when Miles Davis was playing?
The word "cool" used in aviation shpould in my book only used regarding temperature.....
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To clarify, rgsaero, James Brown would never be caught setting the altimeter against the GPS as doing so provides no satisfactory datum for either separation or terrain avoidance, and a more reliable altimeter subscale setting should always be accessible. Curtis Mayfield would consider it unwise and poor airmanship, potentially to the point of being dangerous. I hope you're cool with that.
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Not wishing to get into a fight with you again - but you did.
You pointed out many of the problems with altimeters.
You pointed out many of the problems with altimeters.
Altimeters do have errors, just as every other instrument in the cockpit lies to the pilot, from the reliable magnetic compass to an attitude indicator. All have errors.
None the less, there is a very valid reason why GPS is not the method for setting altitude, as we have seen, and why the barometric altimeter continues to be the method of choice, and of law.
Made the case for using GPS? Hardly. More like a ringing condemnation for an attempt to do so.
Pilot error. Pure and simple. No condemnation on the use of the altimeter, and really quite irrelevant as a comparison with GPS...as GPS wasn't invented at the time.
One could point to the use of advanced GPS and FMS equipment that's enabled crews to put airplanes into hillsides due to a loss of situational awareness, too.
The altimeter in the case cited wasn't at fault, and didn't fail. The crew did.
One of the things we do today before anything else when given a climb or descent clearance is set the altitude alerter. This ties in with autoflight functions, but at it's basic level provides an audio annunciation when approaching within 1,000 and 300' of the target altitude, as well as giving an audio alert for altitude deviations.
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The altimeter in the case cited wasn't at fault, and didn't fail. The crew did.
The job of designers is to take the pilot out of the equation for exactly this reason.
Forgetting to adjust the altimeter, forgetting to arm the pressurisation, forgetting that pilots are human, forgetting why people wish a lot, are all good reasons and some why I supect we will end up with a more autonomous mechanism for altimeteres - I quite like IO540s suggestion.
Maybe one day. In the mean time it wasnt me making the case for GPS, rather you explaining why the altimeter is not perfect.
No, I did not
Last edited by Fuji Abound; 16th Sep 2010 at 11:16.