Two altimeters

[Gertrude the Wombat]

There's posh - I'm used to only having one.

So, what do I do with them? Clearly there are many possible choices of combinations (a) as to what to set the second one to and (b) which one to fly with.

For example, (1) always have QFE on the left and QNH on the right for local area/circuit, or (2) always have the one you're flying to on the left and the other of QFE/QNH on the right, or (3) always set them both the same so they're checking each other.

I gather there's no standard practice here, so one has to develop one's own habits. Which means developing one's own habits in such a way as to make it harder rather than easier to forget which is set to what, and harder rather than easier to find oneself flying off the wrong one.

(Not that it makes a lot of difference when the altitude of your airfield is measured in inches, but I do go to slightly less flat places sometimes.)

What do people do?

[ModernDinosaur]

I do different things depending on the phase of the flight...

* Below the transition altitude and outside the circuit, both are on QNH.

* Above the transition altitude, the "primary" altimeter (usually the one to the right of the AI) is on 1013, the "secondary" on QNH.

* In the circuit, the primary is on QFE, the secondry on QNH.

As for "standard" practice - no-one ever told me anything!

Cheers,

MD

[MasterCaution]

Below transition altitude for a x-country flight I'd take off with QNH on the primary and QFE on the secondary. For cruise below transition I put QNH on both, above transition 1013 on the primary and local QNH or RPS as appropriate on the other. On approach I use QFE on the primary and QNH on the secondary unless the published circuit level is given as an altitude in which case I reverse (means I always fly the level on the primary regardless of what type of level it is).

Unless of course I'm flying CB and don't want neck ache :)

MC.

[bookworm]

At the risk of being provocative, most people who fly around with two or more altimeters have long given up setting QFE on any of them. :)

I'd suggest that below the transition altitude you set a suitable QNH on both. Above that, the choice would seem to be between 1013/QNH or 1013/1013.

There are arguments both ways, but a potentially fatal altimeter error is likely to be detected faster if both altimeters are set to the same thing. Having QNH on the second gives you rapid access to your altitude, which might be useful in an emergency.

[Tinstaafl]

That's what I do, Bookworm, usually 1013/QNH when above transition or cleared to climb to FL. Below transition I'll **always** have at least one on QNH. :ok: Bug ger using QFE for a joke.

[Rod1]

I set both to QFE. The one, which is lower, I then consider the master and set QNH or 1013 on the other depending on what I am doing. If the difference is more then 75ft it is almost certain one is out of tolerance.

Rod1

[Cusco]

Our Arrer has two altimeters and I'd always kinda assumed that the one closest to the transponder was the one that supplied Mode Charlie with altitude information.

But now I'm not so sure........

Anyone know where transponder gets its info from? Presumably if one altimeter only in your A/c , *that* feeds the Txponder, hence the need to keep to regional QNH en route.

Safe flying

Cusco

[flyingfemme]

I'm open to correction - but I thought mode C always used 1013.

[andyb79]

as far as im aware mode c recives its info at 1013 regardless of what the subscale is set too

[Pigasus27]

Why have QNH and QFE at all? In the US they seem to manage perfectly well without all this Q nonsense.

[Flyin'Dutch']

TXPDR gets it altitude info from the altitude encoder; that is a bolt on piece on most transponders; sure some will have built in ones.

They are indeed set to 1013.

FD

[bookworm]

Anyone know where transponder gets its info from? Presumably if one altimeter only in your A/c , *that* feeds the Txponder, hence the need to keep to regional QNH en route.

There are (at least) two possibilities. Either there is a blind encoder connected to the static system idependent of the altimeters, or one of the altimeters is an encoding altimeter with the encoder built in.

In neither case does the setting on the subscale of the altimeter make the slightest difference -- the transponder always reports pressure altitude from whichever source. In the case of an encoding altimeter, the subscale setting only changes the position of the hands, not the encoder output.

[Tinstaafl]

The USA does indeed use QNH. They just call it 'Altimeter' & use inchesHg.

[englishal]

A recipe for disaster in my opinion. I came across QNE the other day, the altimeter reading of the airport when the subscale is set to 1013......

I learned a little trick several years ago, it entailed looking up the airport elevation BEFORE getting airborne . This allows me to land at an airfield which is not at sea level WITHOUT (yep, thats right) setting QFE. :D

I suppose I'm missing out on a lot of the altimeter twisting fun though. Just think, on a cross country, you first set QNH on one altimeter, and QFE on the other. You transition above the Transition altitiude and you now twist one to 1013, and set the regional QNH in the other. but hold on a minute, you now need to cross a class D zone. This may require the 1013 altimeter to be set to a local QNH, as the transition level may rise to > your FL. So you exit the zone, and reset regional QNH, but now you want to transition a MATZ. You are passed QFE, which you must now set, until you are clear and then you go back to regional QNH. Ok, you're now descending into your destination, you are passed QNH and QFE which you set. the weather is a bit crappy, you're turning base....hmmm, did I set QNH or QFE........

:D

[PhilD]

I'm with englishal on this one - the less playing with altimeter settings the better - I'm sure that there are more important things to do and sooner or later you will just confuse yourself.
The more flying I do in the US the less I am convinced that QFE is necessary. Just lookup the airfield elevation and use QNH.

Below transition altitude it's QNH/QNH for me. Above it's 1013/1013. No room for confusion and it makes the cross check very easy.

[Aussie Andy]

Someone said:Why have all these Q-codes anyway?Its obviously a throwback from days when a/g communications relied on the Morse code. See http://www.kloth.net/radio/qcodes.php for a very complete list!

I suppose the advantage of continuing to use such codes is that even when used verbally they provide clear and concise meaning - especially helpful in non-English mother-tongue areas of operations. For instance, in France you will often be told the "Fox-Echo" by which they mean QFE. Think also of uses such as QDM / QDR etc. - once you've learned them, its clear and less of a mouthful via r/t.

I suppose though that the heart of the question posed is whether or not it really necessary or helpful for us to use a combination of airfield and sea-level datums at all: why not just one seal -level "pressure setting" as in the US (and Australia and probably other places for that matter)? I have heard it argued that for instrument flight you are better off using QNH alone anyway - just do the mental maths for circuit and decision heights etc., avoiding the need to do similar mental gymnastics on the go-around, arguably a time when your workload will be higher.

This argument seems to make sense to me, and I find that it really isn't a hassle relying on QNH alone when in the US for example. Yet I always settle right back into using QFE when flying around here...

Would be interested to know what others think?

Andy

[Tinstaafl]

In the UK if I'm offerred a QFE I specify that I require QNH. Until & unless surrounding terrain is referenced to QFE I'm sticking with QNH.

[tmmorris]

It's your right to request QNH instead of QFE, of course, but I wonder - if there's no point in QFE, why are the military so keen on it? I know they*'re a bunch of traditionalists, but surely there must be some point?


Tim


*(OK, technically 'we'...)

[UL730]

Two altimeters - There's posh - I'm used to only having one.


Posh is 3 - you've forgotten the rad alt ;)

Handy safety tip on precision or non precision approach is to have both alts set to same reference datum ( preferably QNH ) for FAF and cross check .

Worth knowing yer tolerances - rarely do both alts totally agree ... then position error....oh dear..... :ooh:

[Kingy]

Kingy's pet hate no.63


I fly an aircraft with only one altimeter that, when I'm flying solo, can't be reset in flight (it's in the front, I'm in the back!). with no radio I set it to my known strip elevation to give 'my' QNH.
Just add the field elevation on to your curcuit height at your destination (it's on the map for crisesakes!) and bobs your uncle.

I must admit it can get a little 'fun' adding or subtracting the 30fts on the rare occasions when I have to use the radio to alight at a larger field though.... :eek:

But generally excessive altimeter twiddling is needless for uncontrolled VFR flight in the UK. In fact, I feel there is a safety issue with everyone flying on the same regional.

You can have your thread back now gents!

Kingy

[Flyin'Dutch']

rarely do both alts totally agree ... then position error....oh dear.....

Position error relates to ASIs not ALTs.

FD

[UL730]

Mr FD

Respectfully suggest a quick peak at PANS-OPS Doc 8168 -OPS/611 or for easy reading JAR-OPS 1 - Minima Specification - say for Cat 1 Mins and review PEC (Position Error Correction) For really snoozy reading - pressure instruments ...... installation errors....static tubes....alignment......zzzzzzzzzzzzz.

Gen in POH for A/C type. Stick to NDB approaches until fully satisfied on this matter:O :ok:

[Tinstaafl]

There most definitely is position error for altimeters. Oz requires (used to require?) PEC to be applied to precision approach DA. If PEC tables not available for the a/c then a minimum of 50' to be added to DA.

[UL730]

Then there's altimeter temperature error proportional to ISA deviation. Corrections mandatory when a/d at or below freezing. Then chuck in a super low QNH 945 - makes yer wonder how the whole show keeps chugging along:8

[tmmorris]

UL730 - that, of course, is what QNE is for.

If your altimeter won't set 945, you set 1013 and request QNE from ATC. Say they report 'QNE 1314ft'. Then you pretend the aerodrome is at 1314ft, adding the heights on the IAP to this to give altitudes to check on finals.

Personally I like Q codes. There are several lists on the web and I'm sure there'll be a thread around somewhere. One you still hear very regularly is QSY, of course.

Tim

[Wot No Engines]

Having learnt to fly gliders in the UK, the norm was to set QFE and leave it - a gap in basic training which never really covered x/c flying. I generally flew on QFE as this is what I was used to. This was most useful at the top of a winch launch in guiding me to how long I had to find a thermal.

Flying x/c, I noted the offset above or below for both QNE and QNH - I still used QFE for the reason above. The rest of the time, I then generally ignored it except for avoiding airspace. The only other key information I wanted was in calculating a final glide - always to the starting field, so QFE is best - pressure is generally stable when a thermal x/c can be flown. I do this to double check the electrics that base the final glide on GPS altitude - no idea what this is based on, but as long as I calibrate by giving field elevation of start field, I seem to get reliable results.

Having moved to Oz, I now am basically forced to use QNH - I can't set QFE on any altimiter I have used here yet as the field is too high (2200') - not a problem in the UK - highest field I flew from was about 1000', but it is here and is in the US. This is probably the main reason for the difference.

As to having 2 altimiters, for the UK, I would set 1 to QNH and 1 to 1013 and leave them.

In Oz, if my understanding is correct, both would have to be set to QNH until transition at 10,000', when 1013 has to be set.

[Keef]

Back to Cusco's question - if the transponder gets its info from an altimeter on your panel, that altimeter will have the word ENCODING printed on the dial (well, all the ones I've flown did).

I was a radio amateur from a tender age, forced to use morse cos I wuz too poor to buy a microphone and build a modulator, and I think Q codes are wonderful .

QNE is worrying - it means "field elevation shown with 1013 set" (sometimes called pressure altitude), but I often read postings from people who think it's the Q code for "set 1013" rather than QNH or QFE.

Like many, I was brought up to use QFE for takeoff and landing and QNH for everything else. IFR training in the USA soon cured me. QNH on both, and cross-check. Or 1013 on both, ditto. Close to trans alt/level and I'll have one of each. Each to his own, I suppose.

[Obs cop]

With the rare delights of 2 altimeters, I revert to the KISS keep it simple stupid approach.

In short, I always keep both altimeters on the same setting be that QFE, QNH, 1013 etc. It allows me the joy of crosschecking at appropriate times, particularly during approaches.

Furthermore, you can only fly with reference to one so why add confusion by having them all on different settings. Using different altimeters just adds complication to the instrument scan, whereas it makes more sense to use a primary and backup method.

IMHO of course,

Obs cop

[Gertrude the Wombat]

Thanks for the comments folks - I can see that I'm going to have to make my own mind up :) - and then try to remember what it is I've decided :(

[UL730]

There is a delightfully coloured pink AIC issued by those clever chaps at AIS that discusses instrument pressure settings in conditions of abnormally low atmospheric pressure settings.

When these conditions prevail, perceived wisdom is, unless you have an altimeter suitably adorned with a setting range that accommodates a QNH below 950 and IMC is likely to be encountered, which is prob 90, given the instability in the atmosphere at these barometric levels – take off should not be made.

If VMC prevail for the whole flight – set system to lowest possible setting and chug along with alacrity.

Instrument let downs in UK on QNE, in IMC because the altimeters can’t read the correct QNH might at best induce a cosy chat with insurers over tea and biscuits. At worst – a more profound meeting with those cheerful chaps at the Coroner’s Court who constantly complain about unnecessary workload. :8

[On Track]

Is Britain the only country where QFE is still used?