Flying with vacuum failure

[Flight Safety]

Two days ago there was a Bellanca 17-30A accident a few miles from my home, that killed both occupants and burned 2 houses. Shortly after takeoff from Addison airport, the pilot radioed that he had instrument panel failure and used the word "gyros" 2 or 3 times. Then the plane crashed into 2 houses just to the left of the extended runway centerline, roughly 5 miles from the airport.

I don't have exact weather data yet, both it was very humid and foggy that morning. Visibility was reported to be 7/8 of a mile at the airport at the time of the accident. I drove to work that morning at 8am, and the ceiling at 8am (a few miles from Addison) was low cloud base at about 250 feet judging from the "invisible" upper half of the high-rise building I drove to. It wasn't much better by 10am when the accident occurred.

The cause of the accident is still unknown, but it's believed that the pilot was in the low cloud base when he radioed about the instrument panel failure.

Assuming the pilot was inside the low cloud shortly after takeoff, I'd like to ask everyone how you would fly inside a low cloud base if you were to experience a vacuum failure with the loss of the attitude and heading indicators. It's not known if a vacuum failure actually occurred, but this accident caused me to think of this possibility, especially given the circumstances.

(Edited to add the time of the accident, which I meant to include.)

[S-Works]

Partial panel, using the turn coordinator, ASI, DI and VSI. Partial panel work is hard especially if you are out of practice and a vacuum failure so low would have made it hard work to transition to partial panel quickly and effectivly.

It pays to practice partial panel skills frequently if you are instrument flying.

It could well have been the case that this person was not in practice and the failure so soon after take off overtook him with fatal results.

As I recall there was a very similar case in the UK a year or so ago that was put down to the same problem. Vacuum failure on take off and the pilot was unable to control the aircraft and crashed.

[mad_jock]

This skill is covered in the partial panel work during IR training.

You use the turn and bank with reference to your compass to maintain wings level.

And vsi cross referenced to ASI to remain level or hold a climb rate

The whole trick is to remain trimmed and only alter 1 thing at a time.

As with all flying its important to aviate navigate communicate

So if you did have a vacum failure, which is actually pretty hard to spot if you are concentrating on other things.

Its important to fly the machine and to establish yourself above MSA, wings level, and trimmed for level flight pointing in a direction which means your not going to hit another plane (ie controlled airspace) or hit anything on the ground.

Then tell ATC, personally i would declare a mayday, but even if you didn't i am sure that ATC would give you the same service what ever you declared. They would then try and get you somewhere to land or into VMC.

In a failure just afteroff you would maintain wings level then by small adjustments of pitch hold your climb speed. Then unless your going to hit something, climb straight ahead.

RT " MAYDAY MAYDAY MAYDAY G-XXXX Instrument failure climbing staight ahead to y000ft standby"

The standby is important because otherwise ATC will come back to you and start asking questions when you should be putting your full attention into flying the machine. If they do start asking questions which are not critical to the situation ignore them. ie whats the number of POB and the like. But if they say give you a flight safety instruction then respond.

Then once you are above MSA and trimmed and settled. Call ATC again and with there help decide what would be the safest option.

As with all these skills currency plays a big part to the end result. Last year I hadn't flown IFR for 6 months, my scan, and handling were utter pants. After a not very impressive trip I made sure that got at least an hour a month in IFR and during the trips practised partial panel (but not the unusual attitudes stuff) but to be honest even that wasn't enough.

Its all covered in the instrument Thom book.

The contents of this post in no way are meant to comment on the incident.

And my condolences to anyone who knows the deceased.

MJ

[Evo]

A vacuum pump failure in IMC leading to loss of the AI and DI has got to be close to the top of my list of nightmare scenarios. Limited panel skills are taught and they're undoubtedly important, but you need to recognise the failure too - and that's my nightmare.

All you can do is to try and learn to constantly cross check what the instruments are telling you. If the DI says you are turning, does the turn coordinator agree? Straight and level on the AI, are the altimeter/VSI and ASI stationary? Do this all the time and you may catch an instrument failure before you follow it in to trouble. It's very hard though.

[Flight Safety]

Evo, that's where a warning light system comes in handy. In the late model 172S POH that I have, the system has 2 engine driven vacuum pumps. Each pump has it's own vacuum sensor upstream of the tee manifold. When either pump fails, a light (L VAC or R VAC) blinks for 10 seconds, then steady on after that. The light lets you know immediately that there's a pump failure.

Then of course there's the vacuum gauge to go along with the warning lights.

[SquawkModeA]

All that has been said about partial panel is true. With my own (limited) experience I'd like to stress that you should never, on partial panel, try to make more than one adjustment at a time. Learned that lesson the other day when I did a partial panel lesson after about six months off the instrument flying. Spent five minutes trying to hold a fixed descent rate with wings level - without success.

Then gave up on the descent rate and concentrated on holding the wings level. That worked things out.

[englishal]

During my CPL training, my instructor failed the gyros, and as we were above a layer of IMC we shot the ILS proper partial panel, in IMC. To make things more complicated, just as we entered the cloud (stratus layer about 3000' thick) he failed an engine (ME aircraft). It was bloody tricky keep the aircraft right side up, trying to identify a dead engine in IMC with no gyros, with the TC wobbling from side to side like crazy as my rudder pressure varied is next to impossible.

These were sudden post-it-note "failures". We experimented in a Sim afterwards, with the instructor flying the "aircraft". We gave him a slow vac failure in IMC (and a few other failures :D) which almost ended in 'disaster', and probably would have in a real plane....

EA

[FlyingForFun]

I fly an aircraft with a slightly dodgy Attitude Indicator. Since the aircraft is limited to day VFR in any case, we haven't replaced the instrument, and over the course of a year or so I've got used to it. However, at first, it was extremely disorientating having the artificial horizon doing something different to the real horizon - and that was in good VFR conditions.

Personally, I doubt that very many pilots would be able to recognise such a problem, and then handle it safely, unless they'd seen and done it several times before. When you're that low, by the time you've identified the problem I suspect it would be too late (although in this case, it sounds from what you say of the RT that the pilot did identify the problem). Partial panel work that you do for the IMC rating is not enough, IMHO, because there is no requirement to recognise the failure. The only way this could be taught properly would be on a simulator (or else on an aircraft where the vacuum pump can be turned off by the instructor - and although someone may correct me on this I don't think that's legal).

Just my personal opinion, I don't have any stats or documents to back this up I'm afraid.

FFF
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[IO540]

FFF

Partial panel work that you do for the IMC rating is not enough, IMHO, because there is no requirement to recognise the failure

I don't believe this, because even the most knackered just-about-IMC-capable spamcan will have an AI (vac) and a TC (electric). The pilot is taught to scan both of these, and if flying something this basic he ought to be aware that he must really watch things.

Anything less should not be flown in IMC at all, because a vac AI fails gradually and it isn't at all obvious until you are upside down.

In any plane which someone might be flying in IMC for real, there should be a vac warning light which should be obvious when it lights up. There will also be a second (electric) AI which you also keep an eye on.

I suppose that incidents where vac failures cause accidents are where the pilot has such a high workload that his scan goes down the pan (easily done if there is no autopilot for example) or where he is flying a plane which isn't up to the job. None of the planes in which I trained I would fly in IMC myself - the workload is very high. Even a non-slaved DI which drifts 10 degrees every 5 mins (pretty normal for a spamcan) increases workload drastically.

[A and C]

One of the things that I have found when teaching the IMC rating is that some people can fly the aircraft better on partal panel than full panel in the early stages of there IF training.

I can only think that they have less think about with the AI and DI failed !.

The real danger of vac system failure is in the fast identification of the failure as the gyros lose ridgidity this requires a very good panel scan to pick up before you have to try the recovery from unusual attitudes.

[alphaalpha]

An interesting subject. Wasn't it failed primary insruments in IMC that caused the 747 accident near Stansted a few years ago?

I suggest you shouldn't assume that failed gyros can only cause problems in IMC. Flying in poor VMC (very hazy, few k vis; 'fishbowl conditions'), you will use the instruments a lot to keep orientated, particularly if instrument qualified. Under these VMC conditions you may well be less alert about your checks, and it's easy for a vacuum failure to creep up on you without warning. This happened to my wife and I a while back, and it was very disorientating, despite being able to see something of the ground.

IO540 mentioned the autopilot. Mine derives its signal from the TC (it's a two axis type). Presumably you wouldn't want it to try to follow the heading bug on the failed DI, so it would have to be in wings-level mode or tracking a VOR or GPS.

It's important to cover the failed instruments, because it really is very hard indeed to ignore them.

Whilst I agree about the importance of keeping the partial-panel skills current, I would also enlist the help of the GPS. It is much easier to use the pseudo compass screen on the GPS than it is to use the conventional aircraft alcohol-filled compass, with all its errors - particularly if you have to fly a partial-panel approach. An IFR-certificated GPS is obviously preferable, but without DI and AI in IMC, you have an emergency on your hands and should use any tool that can help.

Similarly, if you lose your pitot/static system in IMC, the GPS will be of great help with altitude and groundspeed.

I have made a new year resolution to practice loss of gyros and pitot/static using the GPS to help.

AA.

[style with a smile]

I agree with alphaalpha about using a GPS to help - the garmin 196 in particular has that VFR instrument page which must be of some help on the "DI" and "Altimeter" they have there, though not sure using GS is a good idea. GPS gets a lot of stick - ie should only be used as a backup etc etc, which is true, but I think it could certainly help once a vaccum pump failiure has already been identified

[FlyingForFun]

GPS gets a lot of stick - ie should only be used as a backup etc etcAbsolutely... but if this isn't a time to be using backups, then I don't know what is!

FFF
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[IO540]

alphaalpha

A lot of the time, an AP is no good at all with a vac failure. I have a vac primary horizon and an electric TC and DI but the AP uses both the vac horizon and the electric DI!

Also, when tracking a VOR one still has to fly a heading primarily so I don't think any AP will be able to track a VOR if the DI has gone...

Re GPS as a backup... you get a LOT more out of a GPS if you use it as a primary instrument, concurrently with the others, than if you start using it only when everything has turned to ****.

[alphaalpha]

IO540:

I have an STec 20 and a GNS430. Your post has prompted me to start by reading the manuals to see how the STec gets its signals when following either the GPS or VOR in NAV mode. I assumed the autopilot got a 'track-error' signal from the GPS/VOR and the computer in the TC generated the turn command. As you suggest, it's better to know BEFORE things start going wrong. Although it's manatory to carry the manuals, you really can't read them when you suffer gyro failure in IMC!! ;-) I'll post more in a few days.

Purely personally, I agree that a practical reality is to use GPS as primary guidance and confirm that all is well using map or conventional nav instruments. This is what I do and this is the way I was advised to do NDB and VOR non-precision approaches by my IRE -- using an IFR approved GPS with a current database, I hasten to add.


Style:

I have not used the Garmin 196 (Is this the new one with visual screen representations of instruments?). Do you get a rapid screen response to changes in aircraft attitude and performance? Is it sufficient to use if the gyros fail, or are you better to use conventional techniques?

I have checked my GPS altitude readout and found it typically agrees with the altimeter within 50 feet when I have a good satellite constellation (I have an external aerial). There is also good agreement at typical IFR rates of descent, around 500fpm. The GPS keeps up well. Hence my preparedness to use GPS if I lost the pitot/static.

So far as using GPS groundspeed is concerned, remember if you lose your pitot/static in IMC, the back-up is to use "Power + Attitude = Performance." This is pretty imprecise. If you have been flying for a while before pitot/static failure occurs, you will have a good idea of the wind and of the difference between groundspeed and indicated airspeed. You use this to help you derive airspeed from GPS groundspeed. This helps to confirm that you have selected the correct attitude and power to get the performance you want.

In my long-winded way, I am trying to say that if you lose your gyros or your pitot/static then use all the remaining tools at your disposal!

AA.

[FlyingForFun]

If you lose your pitot/static in IMC, the back-up is to use "Power + Attitude = Performance."I would have thought if you loose your static, the first thing to do is to switch to the alternative static source. If that doesn't work, or you don't have one, the next thing to do is to break the glass on the front of the VSI.

If you loose your pitot, the only instrument which will be affected would be the ASI, so you can fly fairly accurately by setting the right power, and using a combination of AI and Altimeter/VSI to set exactly the right attitude.

Just curious on your thoughts on this one - I'm probably about as inexperienced at actual instrument flying as it's possible for an IMC-holder to be, but I would have thought that it's very rare that "Power + Attitude = Performance" really is all you have to go on if you don't have the use of a GPS?

FFF
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[englishal]

If you loose your pitot, the only instrument which will be affected would be the ASI.
Ah but again it is recognising the fact. In fact it nearly killed a friend of mine. He was giving another friend a joy ride, and this particular person ...how shall I put it.....had obviously eaten all the pies :D

In short after takeoff on the climb out, un beknown to my mate the ASI started to give false readings, and of course as he climbed the ASI dropped causing him to increase power and lower the nose. He thought he'd made a serious mistake in weight and balance calcs, and ended up looking for a field to dump it in. I think it was GPS that finally saved him, he figured that with a GS of 150kts, there was no way he could be doing 75 kts IAS. The Pitot had become blocked.

There was a case saveral years ago of a 727 (I think) which crased and killed everyone onboard. Turned out the Pitot tube became blocked, and as they climbed higher the ASI reading increases, so they kept pitching up and ended up stalling.....

Cheers
EA:D

[alphaalpha]

FFF -- You're right, of course. But some pitot/static problems will not be helped by using alternate static and I suppose that's what the Power + Attitude = Performance training is about.

I will shoot down my own argument. Imagine you have encountered significant icing conditions in an aircraft not so equipped. The static vent freezes over and you are smart enough to recognise this but you forget all about alternate static sources. With an increasingly heavily iced airframe, P + A will not equal the performance you normally get. Setting cruise power and normal attitude will lead you into problems..... and the GPS here would be a life-saver. But then you shouldn't have been there in the first place!

AA.

[Timothy]

AA

AIUI the STEC uses the Turn indicator, which is electrical, and is therefore entirely independent of the suction system. If you have a purely electrical a/p that has got to be the safest way to ensure that you remain the right way up until you are in VMC.

There is definitely a lag on the "DI" readout on the 530 (and I assume therefore the 430), which is not surprising when you consider the technology, but that lag is considerably less than that on the compass, and is quite useable if you make slow turns.

I would be very cautious before you assume that the altiutude readout on the 430 is independent of the static system. Certainly on my installation I have a blind encoder driving the altitude readout on the 530. You should check with your avionics engineer before you assume that the 430 is a backup to the static system.

Will

[Flight Safety]

NTSB Identification: FTW04FA052
14 CFR Part 91: General Aviation
Accident occurred Thursday, January 01, 2004 in Dallas, TX
Aircraft: Bellanca 17-30A, registration: N4104B
Injuries: 2 Fatal.

This is preliminary information, subject to change, and may contain errors. Any errors in this report will be corrected when the final report has been completed.

On January 1, 2004, at 1004 central standard time, a Bellanca 17-30A single-engine airplane, N4104B, registered to, and operated by a private individual, was destroyed when it impacted residential structures shortly after takeoff from Runway 15 at Addison Airport (ADS), near Dallas, Texas. The instrument rated private pilot and his passenger were fatally injured. Instrument meteorological conditions (IMC) prevailed and an instrument flight rules (IFR) flight plan was filed for the 14 Code of Federal Regulations Part 91 personal flight. The cross-country flight's intended destination was Amarillo International Airport (AMA) near Amarillo, Texas.

According to Air Traffic Control (ATC) data provided by the Federal Aviation Administration, the aircraft departed the Addison Airport from Runway 15 at 0957. Approximately four minutes into the flight, the pilot reported that he "lost his panel." At 1003, the radar contact with the aircraft was lost. The aircraft impacted a private residence in a northeasterly heading, and came to rest in the garage of a second private residence, across the street. The aircraft and both residences were destroyed by post-impact fire. There were no reported ground injuries. Witnesses in the area reported hearing the sound of an engine pass overhead, at a low altitude, and then heard a "loud boom." The impact was hard enough to rattle neighbors' windows. Witnesses reported hearing the noise, but it was too "cloudy" to see anything.

The weather conditions for ADS at 0957 reported winds from 130 degrees at 6 knots, visibility 7/8 of a mile with mist, and the ceiling was 100 feet overcast. The temperature was 17 degrees Celsius, and the dewpoint as 17 degrees Celsius, with a barometric altimeter of 30.20 inches of Mercury.

Things of note, the aircraft departed Southeast on runway 15. The neighborhood where it crashed is just to the left of the extended runway centerline about 5 miles out, but the aircraft's heading at the time of impact was northeasterly. The ceiling was 100 feet overcast at takeoff and the temperature was at dewpoint. The ground witnesses also reported that they couldn't see anything because it was "too cloudy", and I also heard this on the news. Sadly, this is the first accident listed on the NTSB monthly reports for 2004.

[IO540]

WCollins

The altitude readout from a GPS is normally the vertical portion of the 3D GPS solution.

Over the last 200hrs I have always found it (KLN94B) within 100ft of the altimeter - whenever the altimeter was set to a known good QNH; "regional QNH" is frequently well out. This is at few thousand feet or lower; above that the error is bigger but that's most likely due to altimeter errors due do deviations from ISA.

The QNH setting, and the altitude encoder, which an "IFR approved" GPS has is for RAIM purposes. It does not affect the displayed altitude.

For what it's worth I always check the GPS altitude against both altimeters before an instrument approach. It's never been out, but if it was, and both altimeters read the same, I would have to suspect something potentially dangerous with the altimeters.

I understand why "GPS altitude" is unofficial; I am merely saying what I know about it.

[Keef]

Our GNS430 for sure has an input from the altitude encoder that feeds the transponder - because if I power off the transponder, the GPS throws up a warning flag "no altitude input". I don't know if it's supposed to do that, mind.

[S-Works]

Keef, It is!

You are required to connect an altitude encoder to an IFR GPS. In the event of an encoder are static failure you can switch the Garmins 430/530 back to GPS derived altitiude.

[IO540]

bose-x

Are you 100% sure that the altitude displayed on a GNSx30 / KLN94B is anything to do with the altitude encoder connected to the GPS?

As I posted higher above, the encoder is a RAIM requirement, and should generate a RAIM warning if there is something like a 200ft diff between the GPS solution altitude and the output of the encoder.

For this to work the encoder needs to know the QNH, which is why the QNH has to be entered.

My GPS ('94B) doesn't display the GPS altitude unless it gets a signal from enough satellites, more than 3 I think, and even then it fluctuates by say 50ft until it picks up more of them. If it was derived from the altitude encoder it would not do that.

[S-Works]

For an Installation to be IFR certified the GPS has to be connected to an altitude encoder. The GARMIN 430/530 will use the encoder data as the primary altitude indication.

If the encoder failed it would flag up that it has no altitude data and you can then enter the settings menu and switch it back to GPS derived data. The unit is then no longer certified for IFR flight.

You don't need to set QNH or otherwise as the encoder encodes at 1013mb.

[IO540]

bose-x

Due to the importance of this I have just checked it with a Honeywell/Garmin avionics distributor technican who I know knows the stuff, and unfortunately you are incorrect both on the GNS430 and the KLN94B.

The displayed altitude is derived solely from GPS satellite signals.

(The GNS430 can display the altitude encoder output in one of its maintenance pages but that's it)

The purpose of the altitude encoder is solely for RAIM checking. RAIM checking is a requirement aof an IFR approved GPS. The GPS compares the altitude encoder output (corrected for the QNH which on the KLN94 you enter manually) with the GPS-derived altitude and if there is an error exceeding a certain figure it will flag a RAIM warning.

The GNS430 should have a QNH setting too.

So, if the static system failed, the GPS altitude would still work the same. You would get a RAIM warning though.

[S-Works]

Curious, not what my avionics guy told me but I am willing to check!

[Timothy]

bose, IO540

I don't know which of you is right, but I do think that it is vitally important that we all understand this...there won't be time to find out in an emergency.

I'll look it up in the manual (which I have downloaded) if I get the chance later, and post the relevant text, but if anyone gets a definitive answer first (and, I am sorry, but I don't think we can rely on an avionics engineer for a definitive answer) please tell us all!

Will

[2Donkeys]

Interesting thread with a few arms and legs to it.

When in 3D Mode, the Altitude displayed on the 430 and 530 is GPS-derived. Only when degraded to 2D mode by inadequate satellite coverage does the GPS fall back to display externally serialised altitude (such as from a transponder or blind encoder). It is this need for a fallback under conditions of poor satellite reception that drives the BRNAV requirement.

There are plenty of references to this in the Flight Manual Supplement, the Flight Guide and the Installation Guide.

RAIM and RAIM prediction on the 430 and 530 does not appear to rely on any form of external altitude signal (that I can find). A RAIM error is caused by actual or predicted integrity problems caused by the anglular position of GPS satellites in the sky. It is absolutely not generated by disparity in external versus derived altitude.

RAIM is computed based on the Satellite Almanac contained within the GPS and occasionally downloaded from the satellites. It is calculated with respect to the position which is being FPLed/DIRECTed to and the ETA, or the current position at the current time as appropriate.

Barring external factors such as hangars, a RAIM error will of course tend to occur when the satellite reception downgrades to 2D anyway, so that RAIM and the reliance on the external altitude source are occasionally linked... but not quite in the way being implied above.

Hope this helps

2D

[S-Works]

2D's

Thats basically what I understood the situation to be with and there is certainly no QNH setting on my Garmin.

I am busy scouring the manual!

[IO540]

2donkeys

When in 3D Mode, the Altitude displayed on the 430 and 530 is GPS-derived. Only when degraded to 2D mode by inadequate satellite coverage does the GPS fall back to display externally serialised altitude (such as from a transponder or blind encoder). It is this need for a fallback under conditions of poor satellite reception that drives the BRNAV requirement

The KLN94B is IFR approved and according to Honeywell UK is also BRNAV approved and it doesn't work as above. In the absence of a good enough GPS signal, no altitude at all is displayed.

Do you have a reference for the fallback of the GPS altitude to show the output of the altitude encoder instead, to enable BRNAV approval? The general idea is to use an altimeter for that purpose :O

[2Donkeys]

IO540

You are absolutely right about the KLN94B and the KLB90B. In fact both devices resolve altitude in 100 foot increments from an external source and it is this which is displayed by default. This explains the need to enter or confirm the QNH on startup.

However the business of which altitude is displayed is not key to question of BRNAV certification.

BRNAV is effectively a European manifestation of RMP-5. Mostly this deals with required track accuracy, but where this accuracy is met by GPS receiver, it requires a minimum of either:

4 Satellites to be received, or

3 satellites plus an external altitude source.

The reason for this is nothing to do with giving you an accurate altitude readout. Rather, with only 3 satellites, your altitude provides the only acceptable means of unambiguously determining your position. Cast your mind back to your GPS notes and the intersecting spheres of position that are involved in position determination.

BRNAV since it permits flight by reference to GPS is therefore concerned with belt-and-braces nav capability, and the installation standards were set accordingly. Therefore, your GPS requires an external altitude source to be RMP-5/BRNAV compliant. This has nothing to do with the displayed altitude which is something of a red herring.

The BRNAV standards document is to be found on the Eurocontrol site, although it takes a little hunting down. Each member country published their own documents specifying required avionics fits.


2D

[IO540]

2D

I have a feeling we are talking cross purposes

In fact both devices resolve altitude in 100 foot increments from an external source and it is this which is displayed by default. This explains the need to enter or confirm the QNH on startup

The external source altitude is NOT displayed by default. What is displayed is the altitude from the GPS receiver (AUX1 screen). The output from the altitude encoder is not displayed.

The GPS altitude (displayed) is patently obviously not from an altitude encoder because they return the data in 10ft or 100ft steps. The displayed altitude varies in 1ft increments. Its availability is also related to satellite reception so you don't see anything for some minutes after startup and it gets more accurate as more sats are picked up. I can sit there and watch it, and have done so many times, on the KLN94B and also on the Skymap 2.

The current KLN94B user manual does reveal a somewhat obscure feature of the KLN94: altitude alterting (which I have never used and which on mine is disabled in the maintenance pages; I get altitude alerting from the KFC225 autopilot whose altitude reference comes simply from the P1 altimeter) - which does use the altitude encoder and for that the QNH must be set. Furthermore the QNH must be set when doing a GPS approach - you get prompted to enter it 30 miles out, it says. These KLN94B features really need an air data computer to work well and I've never used them. I believe the GNS430 has the same issues.

As far as I can tell, NO KLN94 function uses the GPS altitude; it is merely displayed in the AUX1 screen. This is as one would expect given that air pressure is used for altitude officially, not GPS.

RAIM integrity checking works by comparing the altitude portion of the GPS solution against an encoding altimeter; the idea being that if the GPS computation is wrong in lat/long then it will also be wrong in altitude and this method will pick up the error - this is a fair enough assumption unless the signal is being jammed is a very sophisticated manner.

[2Donkeys]

We may be at cross purposes, but the RAIM calculation method you are suggesting is at odds with the Bendix King manual I have sitting in front of me.

Ordinarily, RAIM is predicted in respect of the waypoint to which the GPS is navigating, and is predicted effective the ETA at that waypoint. Current altitude is only incidentally a feature of that calculation.

By contrast though, altitude is consulted for instantaeous RAIM integrity (that is, when no destination waypoint is being navigated towards). Failure of the altitude encoder will raise the probability of a RAIM warning because the GPS is denied one of the means of fixing GPS position, however, where 4 or more satellites are received, disabling the altitude encoder will have no effect at all on RAIM warnings.

Try it and see. You'll be surprised. There is much more to RAIM than comparing real altitude with GPS-derived altitude.

2D

[IO540]

2D

It sounds like the current version of the (Honeywell) KLN94B manual has been dumbed down relative to the Bendix King one you have. You are probably right in that if receiving >3 satellites, a RAIM error is unlikely to get flagged. Which is probably why bose-x, never having set the QNH, has never seen anything untoward, because most of the time you receive at least 4 satellites (usually >6).

What I don't agree with is where the displayed altitude comes from. It comes from the GPS, purely and entirely. The altitude encoder output is not displayed directly.

I can't speak for the GNS430 but I am told it works the same way.

[2Donkeys]

IO540

We are in complete agreement on the 430/530. I own and use both of these devices so what I wrote earlier, I can say with complete confidence.

On the Honeywell kit I can only quote from the manual, and a little experience using them. Your description is at odds with the manual - but that doesn't prove anything. :D

2D

[bookworm]

Pure speculation on my part, but I imagine that the requirement to enter the QNH for a GPS approach is because of more stringent precision requirements for RAIM in an approach situation.