Setting QNH/Altimeter after GPS?

[AdamFrisch]

I have very little experience with GPS and mainly fly steam gauges. I was wondering if it's frowned upon to set the altimeter after the GPS altitude reading on stretches where no QNH can be obtained?

[Whopity]

Frowned upon! Its downright bloody stupid! A pressure instrument is set to a known pressure or elevation if you have a reference.no QNH can be obtained? How long do you fly for? You have a met forecast, you took off from a known elevation so you have must a reasonable idea what the QNH is or should be. The Altitude on a GPS is the least accurate part of it!
I was once involved in Research flying where we operated to 50 ft over the sea, we used a totally non standard procedure of setting the pressure altimeter to the Radar Altitude as that was invariably the most accurate datum but even then wave height may have produced spurious readings.

[IO540]

http://i101.photobucket.com/albums/m74/peterh337/gps-altitude.jpg

With the 16ft difference, and that is without even using EGNOS, shall we have a discussion of the wonders of the UK Regional Pressure Setting?

:)

[Fuji Abound]

What a good question.

I did nearly 400nm round trip today and was comparing the GPS altitude(twin IFR certified Garmin 430s) with the altimeter (as well as a few other things). It wasnt totally scientific but IO540 is correct, the differences were very small indeed.

430s must report a RAIM error when the function is unavailable but without a RAIM error what tolerances of vertical nav. should be expected and what tolerances are assured (if any). I must do a little research when I have a sec unless anyoen knows the answers.

To answer the question if for any reason I couldnt set a pressure (I cant think why that would be so mind you) I would definetely use the GPS height to calibrate the altimeter and if the GPS height and altimeter were at variance I would definetely want to estabish why.

[mm_flynn]

As we all know the two instruments in IO's picture measure two completely different things.

The GPS is measuring the position of the aircraft, relative to the centre of mass of the earth (i.e. the thing the satellites are orbiting). The GPS then determines where it thinks the surface of the Earth is using a standard model Geoid. More sophisticated GPSs then take into account the local geoid undulation, or even the actual terrain data. The IFR GPS are pretty sharp at determining where they are in 3d, but the surface of the Earth may be in a slightly different place than the GPS thinks.

The altimeter measures the air pressure and converts the current pressure less the set pressure (the QNH) as a distance, assuming you are in a standard atmosphere. On a very cold day even only 5000 feet above the airfield that has provided the QNH, you could be 500 feet lower than the altimeter shows.

The altimeter will be very good for traffic separation as everyone sets the same reference point, but is less good for terrain avoidance on cold days.

A 'camping' GPS may have a very simple geoid model and hence not have an accurate local view of sea level (and hence get the wrong altitude).

However, like Fuji and IO have seen, once you do the temperature correction so you know what the altimeter meant to tell you! an IFR GPS (particularly one with TAWS) will give a very good true altitude

[LH2]

IO,

What mm_ said (minus some confusion between ellipsoid, geoid, and terrain elevation but it doesn't matter).

Very briefly, a GPS measures time differences from which a series of vectors to known positions in a given reference frame are derived, from which the receiver's position in said reference frame can be obtained and compared to other positions such as that of the ground or an elevation datum. The end result is a geometric distance.

An altimeter is simply a barometer with the scale graduated in distance rather than pressure units. The equivalence between the two unit systems is given by a purely conventional model such as ISA. However, its readout being in feet or metres does not change the fact that an altimeter is solely measuring pressure, or more technically, pressure difference from a calibrated reference.

Therefore, two different instruments measuring two quite different things. Sometimes it just so happens that the numbers might about match. :8


To the OP:

In VFR flight, the purpose of using altimeter + QNH is to keep you vertically separated from other aircraft (+ airspace), which are meant to be observing the same convention. If you were flying on an area where everyone used GPS altitude as a reference then you would not need an altimeter at all, but I don't believe such a practice exists anywhere. In the meanwhile, if you are flying somewhere where for whatever reason you cannot obtain a QNH/QFE, then you would just simply state your reference pressure (which you will have picked more or less arbitrarily) along with your altimeter reading on any communication, e.g., "X-XXXX at 5000ft on 1020".

[Torque Tonight]

It's very not cool to set an altimeter against the GPS altitude.

GPS is much less accurate vertically than it is horizontally and so GPS altitude may differ from true altitude by hundreds of feet. With a typical decision height of 200ft AGL on an ILS, that error could be enough to kill you.

Aviation altimetry is based on indicated altitude above a datum. For QNH the datum is sea level but except in perfect ISA conditions the indicated altitude above the sea level datum will not equate to true altitude because of temperature error. Temperature error affects altimeters equally (but of course not GPS) so indicated altitude remains the correct and reliable method of providing separation. Above transition altitude the standard pressure setting of 1013 is used, which introduces a further deviation from true altitude equivalent to the vertical difference between the datum 1013 pressure level and sea level.

In short, indicated altitude and GPS altitude can potentially have significantly large differences. Except in extreme weather, any QNH from within the last 6 hrs or 200 miles will probably be much more accurate than setting the altimeter against the GPS. If you can't obtain a QNH less than 6 hrs old or from within 200 miles you're doing something wrong.

Of course, sometimes the GPS alt will match the pressure altimeter perfectly, just as a stopped clock tells the correct time twice a day.

[AdamFrisch]

It's mainly hypothetical, but I could probably foresee a couple of places where this could happen. One is the northern route crossing the atlantic (Frobisher Bay-Sondre Strömfjord-Kulusuk) where no HF is required but depending on altitude, you're out of VHF range. Or anywhere else in sparsely populated or trafficked areas; Northern Canada, Africa, Greenland, Siberia, parts of Asia or the Pacific.

There is obviously a very hypothetical danger of flying into terrain in a place where rising terrain and lowering temp and pressure conjure if you're stuck on an old altimeter setting.

Or to give a more recent example, I took off from Lydd and went to Bembridge at 5400ft, which is just 100ft below the Class A veil. Only after I'd landed did I realize that any press altitude change along the way that I was oblivious to could have made me bust the A. Not that the QNH's were not available to me had I wanted to, but nevertheless. Sometimes you're busy watching the scenery.

[IO540]

However, like Fuji and IO have seen, once you do the temperature correction so you know what the altimeter meant to tell you! an IFR GPS (particularly one with TAWS) will give a very good true altitudeI think you mean EGNOS or WAAS, but yes that's about it. GPS altitude is much more accurate than most people think, or were taught to think - assuming the GPS is a decent one. All the IFR units contain a table of geoid corrections for the earth's surface so they do display true altitude above mean sea level.

GPS is much less accurate vertically than it is horizontallyNot "much less" if it is a modern IFR certified unit. There are cheap GPS receivers - I have one here, with the SIRF 2 chip - which have a constant 200ft altitude error in Europe.

where exactly he might be that he cannot get a QNHThere are lots of places where the only way to get QNH is to call up ATC at some major airport (and they might not appreciate it) or to dial up the ATIS (if it exists; many big airports in Europe don't have ATIS). In the UK you have the regional pressure setting which everybody knows is usually inaccurate, though it should give you a "worst case" altimeter setting for terrain separation. It definitely cannot be used for vertical separation from CAS.

Europe does not have the American 18000ft transition level. It varies all over the place, and in CAS you can get a different value issued to you anyway.

Everybody, myself included, knows that barometric altimetry is the accepted standard for traffic separation, but I post this stuff just to make the point that GPS altitude is the height above mean sea level to a reasonable accuracy.

After all, all commercial GPWS systems use GPS altitude which they match against a terrain map. They can't use the baro altitude because the system has no way what the local QNH is, and if it did use it it could not be sure whether the pilot has dialed the wrong value in.

I always check the GPS altitude (on both my IFR KLN94 and the Garmin 496 which usually uses EGNOS for additional accuracy) against the altimeter before flying an approach, and if I had a big discrepancy I would re-check the QNH, and if this was still wrong (which I have never seen) then I would head somewhere with an ILS.

[mm_flynn]

I think you mean EGNOS or WAAS, but yes that's about it.My understanding of GPS fixes is that having the altitude assumed to be right gives a quick fix, but once you have a multi-satellite fix it is equally accurate in all directions (so say +/- 50 meters on a bad day - +/-10 on a good day for a non-EGNOS/WAAS receiver). We just perceive 50 meters vertically to be a bigger error than 50 meters laterally (and of course it is if you are trying to avoid terrain ;-)

The much bigger error is if the GPS doesn't have a clear view of where local sea level is on this part of the earth. I believe all of the TAWS ones have an accurate map of exactly where sea level is and how high the surface is above sea level. The EGNOS ones clearly know this well enough to get you to precisely 250 ft above this surface point for the LPV approaches.

A camping one may use the basic ellipsoid (thank you LH2) and be out by a considerable distance.

[Rod1]

The difference between GPS altitude and FL is one of the issues causing problems in the creation of a truly unified collision avoidance system. FLARM uses GPS altitude to both avoid collisions with other FLARM equipped aircraft and obstacles on the ground. Most other systems use Mode C, or a sensor calibrated to 1013, to calculate vertical separation. FLARM (despite being uncertified) appears to work well in Europe, but I have always been sceptical that the two systems could be brought together in one box. The people developing the systems have always clamed there was no problem, but I have not got my hands on one to test it yet!

Rod1

[IO540]

The difference between GPS altitude and FL is one of the issues causing problems

GPS altitude will always differ from the FL because the latter is just referred to 1013mb and cannot indicate any real altitude.

GPS altitude is the only way to make anticollision work, because one cannot rely on the pilot setting (or even having) the right QNH.

[Rod1]

“GPS altitude is the only way to make anticollision work, because one cannot rely on the pilot setting (or even having) the right QNH.”

That is not the future of collision avoidance kit. The future of collision avoidance kit will be FL, it is almost a universal standard, excluding FLARM. “Being different” is only an issue if the manufacturers cannot reliably convert from one to the other. Two manufacturers are launching kit that allegedly does this.

I have configured my EFIS/GPS system and have chosen not to display GPS altitude (there are 100’s of bits of info you can choose from). As it has an altimeter built in, it may be possible to get it to alert if the two are out by more than a specified amount. If I get time I may have a play.

Rod1

[Tim Dawson]

What an interesting thread. I've recently been involved in the development of an aviation handheld GPS and have been pleasantly surprised by the results.

A modern GPS chipset (such as the sirfstar III) contains pretty much the same geoid correction data as many IFR GPS sets on the market. They get a fix quickly, correct the data so its reference is local mean sea level and most importantly, report the accuracy of the fix.

In the case of this device, we chose to put a little flag in the corner of the altimeter to represent the accuracy of the fix. Usually, in flight with no external antenna, the accuracy is within +/- 25ft. If it isn't, it's +/- 50ft. And this is after a short period of time for the maximum number of satellites to be acquired. Just knowing the accuracy of the information being presented is nice.

Of course this doesn't necessarily make it suitable in the context of the OP's question, since you want everybody to be using the same reference point. Let me know when you succeed in getting a bunch of pilots to all do the same thing :E

[soay]

GPS altitude is the only way to make anticollision work, because one cannot rely on the pilot setting (or even having) the right QNH.
For ACAS equipment, such as the TAS600, to work it's only necessary for mode C transponders to be correctly calibrated. The common reference pressure setting is then 1013mb, so vertical distances between aircraft can be easily calculated relative to each other, rather than using height above MSL.

[Tim Dawson]

[duplicate, please remove]

[IO540]

For ACAS equipment, such as the TAS600, to work it's only necessary for mode C transponders to be correctly calibrated. The common reference pressure setting is then 1013mb, so vertical distances between aircraft can be easily calculated relative to each other, rather than using height above MSL.Yes, correct; I was referring to present or future GPS-based solution. The existing TCAS systems all work on Mode C returns, which in turn rely on the Mode C transponder having a working altitude encoder.

It would suprise me if a FL-based TCAS system was proposed even for the very low end of GA, given how many of those people are dead against installing any electrical equipment, on the grounds that they don't have power available.

GPS altitude seems to be a really easy way to deal with this, especially with EGNOS coming in soon, offering vertical accuracy of the order of a few feet or better. So many altitude encoders are 100-200ft off...

With an altitude encoder you have no way of knowing if it is out of calibration, and if the aircraft in question does not have a transponder then the pilot may never realise it is out, whereas a GPS can easily report if there is something wrong with the accuracy of its 3D fix.

[Fuji Abound]

but if encoders ever used GPS altitudes presumably a mechanism would need to be found for ensuring every GPS was set indentically. In other words there would need to be a common standard of IFR approved GPS encoders - quite possibly one would imagine, cheaper, lighter and more reliable than existing technology.

Are solar flares a worry? We have had a period of weak solar activity but history suggests that will not be the case for ever. However it also leaves me wondering if the GPS signal can be interupted by solar activity can mode c / s transmissions and even primary radar be compromised.

Solar flares will disrupt GPS in 2011 - tech - 29 September 2006 - New Scientist (http://www.newscientist.com/article/dn10189-solar-flares-will-disrupt-gps-in-2011.html)

[IO540]

Solar flares may be an issue (though I stopped reading the New Scientist many years ago, having decided a lot of the content is garbage) but there is no "setting" on a GPS. A GPS computes the 3D fix, or it doesn't.

The SIRF-3 chip used in all cheap receivers today is pretty well corrected for altitude - within maybe 20-30ft. The SIRF-2 had the famous 200ft error.

[Fuji Abound]

By setting I meant there would need to be some assurance that each manufacturer was installing the same chip or a chip that performed to the same specs in every unit. In other words in a world of say SIRF4 and GLASNOS5 for their to be unanimty between GPS units any where in the world a standard would have to be agreed that ensured each chip understood the world to have the same shape and would interpret the signal in the same way where ever the aircraft and at what ever height.

Perhaps that is already the case? Would every IFR GPS unit regardless of make report the same height every where in the world if you placed the two units side by side. I dont know?

In theory two encoders would behave in exactly the same way subject to the certified tolerance.

[IO540]

Perhaps that is already the case? Would every IFR GPS unit regardless of make report the same height every where in the world if you placed the two units side by side.

Yes, they should.

[Tim Dawson]

It depends on the quality of signal. If you've just turned the thing on, you'll get an imprecise fix. After time, it gets better and potentially very good indeed. This will vary subtly with antenna type and position.

[Fitter2]

My understanding of GPS fixes is that having the altitude assumed to be right gives a quick fix, but once you have a multi-satellite fix it is equally accurate in all directions (so say +/- 50 meters on a bad day - +/-10 on a good day for a non-EGNOS/WAAS receiver). We just perceive 50 meters vertically to be a bigger error than 50 meters laterally (and of course it is if you are trying to avoid terrain ;-)

For good geometrical reasons, the algorithm which determines height above geoid is separate from that which outputs lat/long co-ordinates. Since the primary interest of GPS users is lat/long, the WAAS/EGNOS corrected lat/long is more accurate than height by a factor of roughly 2. You have a 95% probability of being within 10 meters of the horizontal fix, and 20 metres of height above geoid. The lat/long are accurate enough for GPS approaches, but I would hesitate to rely on GPS height for the flare and touchdown.....................;)

[soay]

With an altitude encoder you have no way of knowing if it is out of calibration, and if the aircraft in question does not have a transponder then the pilot may never realise it is out
Does it make sense to have an altitude encoder without a transponder? I often get asked to state my indicated altitude after being given a squawk code, presumably so ATC can confirm if my transponder is working correctly.

GPS altitude seems to be a really easy way to deal with this, especially with EGNOS coming in soon, offering vertical accuracy of the order of a few feet or better.
I wonder why Garmin decided to implement EGNOS in the old GPSMAP 296, but not in the new Aera 500?

[IO540]

I would hesitate to rely on GPS height for the flare and touchdown

AFAIK all autoland systems use a radar altimeter for the flare portion. I guess the autopilot switches over to the radalt at about 150ft.

WAAS (EGNOS in Europe) facilitates GPS approaches with a synthetic glideslope but the DH on these is still about 200ft (no European ones are published since none exist) i.e. no better than Cat 1 ILS.

[Torque Tonight]

By way of experiment I have sat on the ground at a point of known elevation and compared a number of GPS receivers that I have, including a couple specifically intended for aviation use. The altitudes indicated have varied significantly, and we're not just talking about that known constant 200ft error relating to the geoid model in certain chipsets. GPS altitudes varied with time, and varied between units at the same time. The altitude accuracy was not something I would stake my life or licence on.

No doubt in future GPS receivers will be better able to accurately measure true altitude - but at the moment altimetry is not, and is not intended to be, based on true altitude. We have a well established, reliable and consistent system of altimetry based on pressure altitude. The 'receivers' are robust, simple, capable of very good accuracy, instant reponse, require no power for their core function, etc etc. There are a number of very good reasons pressure altimeters are here to stay for the forseeable future.

Back to the OP, I think it would be very unlikely that you would ever not have access to a good enough QNH/RPS such that using GPS altitude was preferable. Blind faith in GPS can stitch you up.

Would I ever refer to GPS alt? Well perhaps if I had a static blockage, no alternate source, no standby alt and I was in IMC, then I would probably let down over the sea until VFR using GPS alt, but I can guarantee that I'd descend the last 1000ft or so at a snail's pace. It would take a lot of bad luck to get into that situation though.

[Fuji Abound]

So here is another one - would you rely on a GPS to judge your speed in say, the approach?

(We all know there are other ways of landing with a failed ASI).

[Tim Dawson]

To judge my groundspeed? Absolutely. But a GPS has no idea of airspeed, which is all that matters in an approach :)

[Rod1]

Originally Posted by IO540 With an altitude encoder you have no way of knowing if it is out of calibration, and if the aircraft in question does not have a transponder then the pilot may never realise it is out
“Does it make sense to have an altitude encoder without a transponder? I often get asked to state my indicated altitude after being given a squawk code, presumably so ATC can confirm if my transponder is working correctly.”

It makes no sense to have an encoder without a transponder as the transponder has to transmit the encoder output. When IO said there was no way of knowing if the calibration was correct, he had probably forgotten that you can ask for a mode c check from just about any radar unit every time you fly if you want to. On modern units you can also display the encoder output on the front panel and compare it with your altimeter set on 1013.

Rod1

[Fuji Abound]

Of course it doesnt but you can get the wind from the tower.

How good a guide would you arrive at?

[421C]

There is a lot of discussion of geoid corrections but a point that has been missed is that there is NO standard geoid used in aviation for MSL (unlike the WGS84 standard for lat/lon, which is used both by GPS and in paper charts). Approach plates, for example, may use a "local" MSL datum derived from a different geoid than the EGM96 model used in GPS. I believe these differences can be significant.

Hence, whilst GPS is great for IFR LNAV, barometeric altimetery is needed for IFR VNAV.

brgds
421C

[IO540]

Nevertheless in the 8 years I have been flying behind the old KLN94, I have never seen it display an altitude more than about 30ft (i.e. 1 millibar) different from the published airfield elevation.

That's a pretty good record, even before one starts to compare it against mis-setting the QNH by 10 points (300ft), flying 1000ft too high/low (due to misreading the analog altimeter) etc.

GPS altitude is a lot better than most people think - unless the GPS is a piece of junk from a camping shop, and even the modern ones of those can be very good. My Garmin 496 (OK, not from a camping shop) is within 5-10ft of the airfield elevation and most of the error is probably the height of the unit above the concrete :)

Of course it doesnt but you can get the wind from the tower.

If there is any real wind, say 10kt+ reported SFC wind, I would be more careful using a GPS for flying the final approach, since there is likely to be a significant wind shear in the last 100ft or so. But it should still be better than nothing.

When IO said there was no way of knowing if the calibration was correct, he had probably forgotten that you can ask for a mode c check from just about any radar unit every time you fly if you want to. On modern units you can also display the encoder output on the front panel and compare it with your altimeter set on 1013.

I didn't forget; I was referring to low end GA not carrying transponders (for all kinds of reasons; real, artificial, or Civil Liberties driven) but presumably still wishing to carry some form of TCAS. If you want to do TCAS on the cheap, nothing beats GPS (i.e. some form of ADS-B) for low lower, simplicity, and accuracy.

By way of experiment I have sat on the ground at a point of known elevation and compared a number of GPS receivers that I have, including a couple specifically intended for aviation use. The altitudes indicated have varied significantly

What models were these?

[LH2]

For good geometrical reasons, the algorithm which determines height above geoid is separate from that which outputs lat/long co-ordinates.

Please clarify. I believe you might be referring to the coordinate transformation stage, from ECEF to whichever horizontal/vertical/combined reference systems the user chooses to have. The navigation solution itself is obtained by solving all four unknowns simultaneously and does not even involve the concept of local vertical (and hence altitude). Also the geoid correction does not enter the stage until much latter, if at all.

The reason why the vertical component is less precise than the horizontal component is indeed geometrical however: as you can only see satellites above the horizon, you have no observations at all coming from below (the ones coming from low elevations are also less reliable due to atmospheric effects), which increases the measurement uncertainty in the vertical direction. Note incidentally that this effect diminishes with height above ground due to the lower horizon elevation amongst other things.

The relative "goodness" of the position solution in different planes/components is given by the dimensionless DOP (dilution of precision) indicators.

Since the primary interest of GPS users is lat/long

Do not generalise please. Timing is a major field of use.

Lastly, the geoidal correction bit, which seems to mystify a lot of people is not a major factor in recreational GPS use, aviation included. I have geoidal correction source code at hand, originally from a consumer grade GPS receiver which is less than 7Kb (7072 bytes to be exact :)) and computes a "rough" correction based on bilinear interpolation on a 10 degree grid. This achieves accuracy in the order of 2-3m worldwide (with the possible exception of the great mountain ranges, I haven't checked). Clearly, that's unacceptable for geodetic or many engineering uses, and for Cat III autolands (where the radio altimeter controls the flare anyway), but for general aviation it doesn't matter a bugger.

[LH2]

Approach plates, for example, may use a "local" MSL datum derived from a different geoid

Geoids and vertical datums are different, orthogonal concepts.

[LH2]

Nevertheless in the 8 years I have been flying behind the old KLN94, I have never seen it display an altitude more than about 30ft (i.e. 1 millibar) different from the published airfield elevation.

Well, post May 2000 you shouldn't expect see differences much bigger than those, and if you do see them it's probably a misprint on the airfield chart or the surveyor got the height wrong (it happens :E).

And if still on the tarmac and you have set your altimeter to the QNH provided by the tower, everything should more or less agree.

But what I understand is being discussed here is what happens when you take off, climb to say FL180 and then compare the GPS against the altimeter reading. In that case you can expect to see quite a bit more (http://www.peter2000.co.uk/aviation/troy/egka-ldsb-profile.gif) than 30ft discrepancy between altimeter and GPS. More like 1000ft-1500ft on a random day over Greece :)

[Spendid Cruiser]

Adam,

It is a jolly good question!

However, I don't really see the point of setting both GPS and altimeter to read the same thing. You should use the most appropriate pressure setting for the altimeter. If not because of the usual mode C and vicinity of controlled airspace arguments, but also because then you have two independent references.

To my mind, two independent references would give you a better sense of accuracy than one. Perhaps you could use significant converging/diverging relationship between the two to give you some information about the environment your flying towards or the accuracy of a fix. I reckon it probably adds to the holy grail of SA.

I suppose, if you're of the mind that the pressure setting is out of date and significantly inaccurate and terrain or some such is a threat then maybe consider separation based on lowest of the two until you have resolved the discrepancy to your satisfaction.

[SNS3Guppy]

With an altitude encoder you have no way of knowing if it is out of calibration, and if the aircraft in question does not have a transponder then the pilot may never realise it is out, whereas a GPS can easily report if there is something wrong with the accuracy of its 3D fix.

With better equipment one can monitor the encoder output, and one can always ask ATC what is being seen.

Ask yourself why RVSM isn't predicated on GPS, but on QNE. Nearly everything flying in RVSM has advanced GPS capability. Never the less, altitude isn't based on GPS, even in RVSM, but on barometric altimeter. We do use an air data computer to correct altimeter at altitude, but none the less, it's baro altimeter use, not GPS, and the corrected altitude invariably disagrees with what's found in the GPS.

[IO540]

climb to say FL180 and then compare the GPS against the altimeter reading

Well, yes, a baro altimeter is going to be pretty useless for determining actual heights, when at high altitude, due to a virtually permanently nonstandard atmosphere.

I have never seen the inside of a real altimeter but believe it contains some kind of logarithmic correction, but that will be a 1st order one only, with the OAT not being measured, never mind corrected for.

An altimeter reads accurately only on the ground, after you have set it to the known elevation of the ground :)

Once airborne, it is OK for vertical separation of traffic provided everybody uses the same sort of instrument (which they do).

[SNS3Guppy]

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.

QNH is perfectly adequate and appropriate for use in light airplanes, and it works just fine.

[IO540]

the air data computer considers temperature, temperature and airspeed and other factors when applying corrections to instrument indications

Which brand of barometric altimeter connects to the ADC?

[LH2]

Ask yourself why RVSM isn't predicated on GPS, but on QNE

Because it's self-contained.

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.

Yup, but that doesn't give you a geometric altitude off the altimeter reading. To do that you need to consider the conditions of the whole column of air between the aircraft and the ground, of which you have no data but at one point.

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?

[SNS3Guppy]

Because it's self-contained.

Because what is 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?

Every one that's used in aircraft equipped with an ADC. It's one of the primary functions of 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?

You're not familiar with true altitude? It's a basic aeronautical term, not a US term.

True altitude is actual altitude. It is calibrated altitude, corrected for temperature.

[Jan Olieslagers]

why RVSM isn't predicated on GPS, but on QNE

Suppose that should read QNH? Mixing up QNH with QFE, perhaps?

[BEagle]

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.

Suppose that should read QNH? Mixing up QNH with QFE, perhaps? 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.

[LH2]

Because what is self-contained?

Altimeters. They do not depend on bits outside the aircraft, same as INS.

You're not familiar with true altitude?

Nope. It might be a standard term; heard and perhaps used it before, but I don't recall ever seeing a definition for it, and since there is no such thing as a "true" altitude unless conventionally defined...

True altitude is actual altitude. It is calibrated altitude, corrected for temperature

Note that the above is still not "actual" (geometric) altitude unless you have a temperature profile for the whole column of air beneath the aircraft [and gravity readings, I might add]. I am not disputing that it might be called true altitude though. I might have to dig out my ATPL notes :bored:

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.

[BillieBob]

No, QNE is correct--it refers to standard pressure, as in setting the altimeter to 1013.25 to fly flight levels.No, it isn't and it doesn't. The incorrect confusion of QNE with the Standard Pressure Setting is, unfortunately, widespread and perpetuated mainly by instructors who are too idle to research the facts properly. As BEagle indicated, QNE is not a pressure setting; it is expressed in feet and not in hectopascals, or millibars, or inches of mercury.

[LH2]

The incorrect confusion of QNE with the Standard Pressure Setting is, unfortunately, widespread

I do not believe there is any confusion as regards the above when you say you're "flying on QNE", or "setting the altimeter to QNE". Granted though, the CAP413-sanctioned use refers specifically to landings, but unambiguous all the same. And SN3 above definitely did not confuse QNE with QNH/QFE as suggested.

(and please let's not have another ******* boring thread about "correct" R/T :bored:)

[SNS3Guppy]

Altimeters. They do not depend on bits outside the aircraft, same as INS.

Meaning what, exactly? If the assertion is that GPS is a better source of altitude information, then I ask again, why is it not used for RVSM?

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?

No, QNE...which is what's set once one flies above the transition level.

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.

[mm_flynn]

Meaning what, exactly? If the assertion is that GPS is a better source of altitude information, then I ask again, why is it not used for RVSM?

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.

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)

[SNS3Guppy]

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.

Actually, that's not correct at all. Barometric pressure, corrected through the ADC is used for RVSM, and relies on reference outside the aircraft. It's barometric pressure, after all.

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.

[mm_flynn]

'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

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

[SNS3Guppy]

As regards the remainder of the post, thank you for so eloquently making the case for GPS altitude.

I didn't. In fact, I think I showed you rather conclusively why it's not used, and can't be risked.

Altimeters don't quit. GPS does.

[Fuji Abound]

I didn't.


Not wishing to get into a fight with you again - but you did.

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.

[IO540]

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 :)

[Jan Olieslagers]

Altimeters don't quit.

Hm. Hmhm. Never? Really NEVER? Hmhmhm.

[rgsaero]

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.....

[Flyingmac]

United Airlines Flight 389 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/United_Airlines_Flight_389)

[Torque Tonight]

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.

[SNS3Guppy]

Not wishing to get into a fight with you again - but you did.

You pointed out many of the problems with altimeters.

You do wish, and often do...and no, I did not.

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.

United Airlines Flight 389 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/United_Airlines_Flight_389)


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.

[Fuji Abound]

The altimeter in the case cited wasn't at fault, and didn't fail. The crew did.

Have you heard the one about the dog and the pilot.

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


The fact that two of us did, and only you did not, would tell me something.

[Flyingmac]

I occasionally fly an aircraft with two sub-scales on the altimeter. One in millibars, the other in inches. Does this mean that it contains mercury?

[SNS3Guppy]

Flyingmac,

The altimeter doesn't contain mercury. Some altimeters do have dual Kollsman scales or windows (milibars and inches of mercury). These don't mean that the altimeter contains mercury, but are measurements that reference mercury. Specifically, they reference the readings that would be obtained from a mercury barometer, or a barometer calibrated to read in "inches of mercury."

Typical measurements for pressure readings include both inches of water and inches of mercury. The standard measurement in inches has been set using mercury, for the simple reason that it's more dense that water and produces easier numbers with which to work. (a column of mercury at sea level is only 30 inches high...about two and a half feet, or a little less than a meter. A column of water at sea level is about 35 feet high, or nearly six meters).

Standard air pressure at sea level will be adequate to support a column of mercury in a glass tube to a height of 29.92 inches. That is, a glass tube, open at one end and placed in a bowl of mercury, will have the mercury pushed up the glass tube by air pressure on the mercury in the bowl, to a height of 29.92 inches. Above the mercury is a vacuum, sealed by the endof the tube. The mercury barometer compares existing or ambient atmospheric pressure acting on the bowl of mercury against the vacuum in the tube, and determines the pressure by measuring the height to which the mercury is pushed in the tube. Mercury is dense, heavy, and doesn't get pushed as high as water...which is why we reference mercury rather than something else like water. This harks back to an old fashioned mercury barometer. These days most barometers don't use mercury, but use electronics to determine pressure. They are still calibrated and read to the standard of mercury, however.

If you fly an airplane with a manifold pressure gauge, it also uses inches of mercury as the reference (which is why, on a standard day, the manifold pressure gauge at rest on the ramp at sea level will read 29.92 inches of mercury). There is no mercury found in the manifold pressure gauge, and none is found in the altimeter.

Have you heard the one about the dog and the pilot.

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.

The is actually for a dog, a cat, a duck, and a brick. A fully equipped cockpit. If IMC and disoriented, and unable to navigate or find up,one opens the cockpit window and tosses out the brick. Follow it down to find the ground. If one doesn't wish to go down, toss up the cat. The cat always lands on it's feet, and so long as it's declawed, where it lands is down. If it's not declawed, where it lands may be up. The duck loves wet weather, and when in doubt and lost in the clouds, toss out the duck and follow it. The dog is there to bite the pilot's hand if he touches anything.

Why not simply make the airplane a UAV...unmanned aerial vehicle? Take man out of the equation completely? Put autothrottles in Cessnas, and create Category III landing capability in the Cherokee? Why have the pilot at all?

We have simple mechanisms in airplanes because they tend to be most reliable. We learn the foibles of instrumentation because all instruments lie to us. Learning the errors is part of learning to fly instruments. Perhaps you have never been IMC and had electronic displays go dark; I have, and I understand perfectly why the revision for a full EFIS cockpit is to go to manual "steam gauges," or instruments. When all the gee-whiz gadgets go south, then one reverts to the stuff that works.

As we continue to see progression into magenta lines, GPS for navigation in all counts, and increasing sophistication in the cockpit, we continue to see a degradation in the skills of the pilot body at large. Increasingly I note that without a magenta line to follow on a display, some pilots couldn't find their backside with both hands, and without automation, some are completely lost. The availability of GPS in the cockpits of even the most simple light airplanes today with quasi-FMS capability does offer some wonderful capabilities, but also heralds laziness in learning and practice.

A company for whom I worked some years ago fired a chief pilot. He captained a medical flight into a remote high desert airport in mountainous terrain. The approach was nearly always flown at night, because night equated to instrument conditions. His altimeters worked perfectly, but he was quite convinced the field elevation was a thousand feet lower than it actually was. He set the alerter to field elevation, expecting to have it ring out a thousand feet above the field.

His charts gave the correct information, and he was flying an instrument approach. He had flown there before, but none the less, for some unknown reason, set field elevation a thousand feet low. A medic who had made the flight many times before caught the error. Once on the ground the medical crew refused to fly with the man, and within 24 hours he was released from employment.

Pilots make errors but GPS wouldn't have changed his view any more than a barometric altimeter. The airplane was quite capable with GPS on board, FMS, electronic displays, etc, yet he still made the error. Until one elects to go completely automated or unmanned, one will continue to have errors. I can tell you, however, that even in unmanned situations, the result isn't as peachy as you might think. Highly sophisticated unmanned assets are often not where they are supposed to be...and in fact are often off their altitude by a considerable value.

We put pilots in airplanes and train for a good reason, just as we continue to use barometric altimetery vs. GPS altitude, for good reason.

[Flyingmac]

Flyingmac,

The altimeter doesn't contain mercury. Some altimeters do have dual Kollsman scales or windows (milibars and inches of mercury). These don't mean that the altimeter contains mercury, but are measurements that reference mercury. Specifically, they reference the readings that would be obtained from a mercury barometer, or a barometer calibrated to read in "inches of mercury."



Guppy. Thanks for the explanation but my question was simply a poor attempt at what we Brits call humour.(Humor). My apologies.
I'm off now to inflate my car tyres to 75 inches of mercury.

[SNS3Guppy]

My family is British. I grew up with a picture of the Queen on the wall. I was harassed and harangued about use of language, and given a very hard time in school because I grew up with an English accent. One would suppose I would be better at reading between the lines...but alas, no.

Generally, unless I am hit squarely in the face with the blunt details, to me it remains invisible.

[mrmum]

Flyingmac;

Guppy. Thanks for the explanation but my question was simply a poor attempt at what we Brits call humour.(Humor). My apologies.
I'm off now to inflate my car tyres to 75 inches of mercury.


Not poor at all, made me chuckle, maybe some judicious use of emoticons would have helped.

[Fuji Abound]

As we continue to see progression into magenta lines, GPS for navigation in all counts, and increasing sophistication in the cockpit, we continue to see a degradation in the skills of the pilot body at large. Increasingly I note that without a magenta line to follow on a display, some pilots couldn't find their backside with both hands, and without automation, some are completely lost.


I know, but it has been done a million times before. Things change. People cant read maps these days because they have a sat nav. Yachties follow green lines because they have no idea how a sextant works and those that do have no idea how to read the stars. Without boasting I reckon I could do a reasonable job of getting you across the pond with nothing more than a sextant. In fact it is still kind of fun just like the navigation competitions we do here but skills change. Garmin and Avidyne both tell you if the PFD and MFD fail the correct action is to land as soon as possible. The aircraft is not going to stop flying, the chart on your lap isnt going to burst into flames, but hey, call AT, ask for vectors to the nearest, and land.

Fact is it takes a lot of time and experience to develop the situational awareness to keep yourself out of trouble in IMC when the pressure is on. How many high time professional pilots have paid the price? Fact is with the scenery painted on a 12 inch MFD life has just got a whole lot safer for the pilot and when it fails because he knows he is blind he is going to call for help. There is many a pilot flying on conventional instruments who at some point realises they may have lost the plot, but makes the mistake of blundering on.

Times change. Inevitably the older we get the less we like change and the harder we find it to adapt - but I dont yet wish we could turn the clock back. :)