Stupid question – altimeters.

[localflighteast]

If this is a stupid question, please ignore and put it down to the fact that I’m at home ill and also suffering from severe flying withdrawal (4 weeks damn it!)

I was contemplating the instrument panel of the trusty 172 that I fly and my gaze was drawn to the altimeter, in particular the little Kollsmann (sp?) window.
Here in the great white north (and the US presumably) it is calibrated in inches of mercury (because metric would be far too easy!). I have heard rumours that the UK uses hectopascals (I think these used to be milibars?).

I know that some people fly N reg US planes in the UK, do they have to change out the instrument? Use tippex on the numbers? Or do some funky conversion in their heads? has this ever caused any issues?

Just curious and again If I’ve blatantly missed the point, be nice to me.

[dubbleyew eight]

not a direct answer to your question but in the US and Australia they change out the instrument.
the australians replace an inches of mercury subscale instrument with a millibar subscale instrument.
take an aircraft from australia to the US and they do the opposite.

[Dash 7 Rider]

Some altimeters have one window for inches of mercury and one for millibars. There are conversion charts, but you would only want to do that for a limited period of time.

On the part about metric being easier. There are only a few countries that use metric altimeters. With the advent of RVSM the 1000' spacing has become a contentious issue. Some countries have come up with a system for issuing clearances in metric while actually flying in feet. While other countries have just decided to issue clearances in feet.

[localflighteast]

my comment about metric being easier was a little facetious I'm a mere student at the moment and one of the things I struggle with is units.
I was pretty much educated in SI units . I find it tricky working in feet, miles and the like because I don't have a good "feel" for the numbers. Obviously I know how many metres in a Km but feet in a mile (either type) is not intuitive to me.

as for the formula that links feet , degrees C and inches of mercury . Well that just makes my head hurt!

[dubbleyew eight]

I too grew up in a metric world.

aviation is feet altitude and nautical miles distance

all of the aeronautical engineering texts I am learning from are in imperial units.

all I can say is that a little piece of freeware written by josh madison called convert.exe is the saving of one's sanity.
Ben & Jerry's newest flavor - joshmadison.com
click on the convert menu item. it is a free download.

[localflighteast]

I have an app on my phone that does pretty much the same thing. It is just taking a lot of getting used to. I'm used to being able to look at a number / answer and have a good idea if it looks about right, finding it harder with unfamiliar units.

Not sure if I'm expressing this terribly coherently.

Anyways thanks for the info so far everyone.

[dubbleyew eight]

nautical miles persist in aviation because a nautical mile is one minute of longitude, 6080feet.
on a World Aeronautical Chart (WAC) come down a line of longitude on the map and each one of the minute ticks is a nautical mile. 60 nautical miles to a degree.

[Mariner9]

Nautical Mile

Bit of thread drift (sorry), but W8's post above about nautical miles is not strictly correct.

A nautical mile is roughly equal to 1 minute of latitude (about 6076 feet). It is defined however as exactly 1,852 metres

[Heston]

...and a metre is
the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second


There are no stupid questions, btw. Only stupid answers

[3 Point]

I think the OP's remark about metric was referring to the units used for setting the pressure sub-scale, not to the units used to measure the Altitude of the aircraft (correct me if I am wrong!).

In some parts of the world a unit called "Torrs" is used as an altimeter setting; 1 Torr is approximately 1 mm of Mercury and 760 Torrs equals exactly the Sea Level pressure in a standard atmosphere (ie 760 Torr = 1013.2Hpa = 29.92 InHg). I have used it when landing at an airport in Russia!

See here if you want to know more ...

Torr - Wikipedia, the free encyclopedia

Modern EFIS equipped transport category aircraft have an option in a menu of the EFIS to change from Hpa to InHg and back again; you change half way across the Atlantic from 1013Hpa to 29.92 InHg. There was no option to choose Torrs when in Russia so we used a conversion table on paper and set the equivalent Hpa setting.

Using Metres in place of Feet to define Altitudes, Heights or Flight Levels is of course common in many parts of the world and again, EFIS aircraft usually have an option but otherwise you just use a conversion table and set the equivalent.

Happy landings

3 Point

[localflighteast]

I was just bitchin about non metric stuff in general.

As I said , this was just mild curiosity about N reg planes in UK. I take it there's no reg that requires you to rip out the altimeter and replace it with a suitable one.

I guess I was just wondering as missetting the altimeter can do all kinds of wrong things. I misset mine once ( out by a whole inch). Of course I picked it up on my preflight instrument checks where the altimeter was definitely not showing field elevation ! A quick flick back to the ATIS set me straight.

[3 Point]

When an N reg plane visits the UK then of course it keeps its altimeter; how would airlines manage if you had to change?

However, if you take an N reg PA28 and bring it here to put it on the G reg I think it then has to have an altimeter in Hpa fitted. I say "I think" because I can't quote the regulation and I may be wrong; perhaps someone else can confirm.

I agree, it's a real pain changing units all the time but it's just the way it is! I frequently fly to the US where I get my fuel in USG, record the uplift in Litres on my tech log and then look at my fuel gauge which shows Lbs!

Always the possibility of an error when making conversions so robust cross checking procedures are very important. You set your altimeter then checked against the field elevation and spotted the mistake; absolutely the right way to operate. Uploading 1,000Lt of fuel when you really wanted 1,000USG could really spoil your day!

Happy landings!

3 Point

[localflighteast]

yeah , I'm thinking small planes here, like a 172 or something obviously airliners would have to physically change out the altimeter. I guess I was just thinking that when I do a cross country flight here , every time I get new info you change your altimeter to the reported setting from the local info , getting it in one set of units and having to do the conversion "on the fly" would be a pain.

Yep, most people with a working knowledge of Canadian aviation history have a good idea of what happens when you get the fuel conversion wrong. Gimli Glider .

[Heston]

Yes getting the units wrong is said to be an issue. In the UK when the altimeter setting is passed over the radio, if the number is less than 1000, the units have to be said. Thus: "QNH 990 hectopascals". It is said that the reason for this rule is that US pilots flying here might mistake "QNH 990" for an altimeter setting of 29.90 inches and thus set their altimeter wrongly. Regretably a large proportion of pilots and ATCOs find it hard to pronounce "hectopascal"

[TheiC]

...if you step up the Cessna product line to find something recent with a model number starting in 5 or 6, there's a little button you may push to swap the datum from HPa to inHg and back, and another one to turn feet into metres.

[3 Point]

Yep,

"there's a little button you may push to swap the datum from HPa to inHg and back, and another one to turn feet into metres"

As I said, most modern EFIS aircraft can do this. I've never flown with a Garmin G1000 cockpit but I'd be surprised if it can't do it too! So that means even a 172 has that capability!!

"getting it in one set of units and having to do the conversion "on the fly" would be a pain"

It's not too much of a pain. You know you're going to have to do it so you have the conversion table ready to go. Get the new pressure setting from the Controller and read it back as he said it, then write it down. Get the table out and make the conversion. If it's a two pilot operation then both pilots convert independently and confirm before setting the new datum. If its a single pilot show then just take care and cross check it yourself before relying on it.

As pains in the ass go it's one I can live with fairly easily.

3 Point

[Discorde]

The logical answer is to adopt metric measurement throughout the industry, so distances are metres or kilometres and speeds are kilometres per hour. Atmospheric pressure units would be hectopascals and standard vertical separation 300m, which is almost identical to 1000 ft. Low-level (high workload) altimeter missets would be reduced by use of a universal Transition Altitude of 6000 metres and eventually by adoption of GPS altimetry.

[Heston]

The LOGICAL answer. But we are talking aviation it ain't gonna happen.


Actually after a while all the funny mixed units we use become second nature and not a problem at all.

[Gertrude the Wombat]

I know that some people fly N reg US planes in the UK, do they have to change out the instrument? Use tippex on the numbers? Or do some funky conversion in their heads?

You could just twiddle the knob until it reads airfield elevation before take-off, and assume it won't change much during the flight, and land on QNH when you get where you're going.

[this is my username]

If you are flying an N-reg aircraft with a mechanical altimeter in the UK/Europe then you just need to carry a small conversion chart to get you from HPa to inHg.

Or you could always ask ATC, as one pilot (allegedly) once did::

"N12345 descend altitude 3,000ft QNH 1022"

"Could I have that in inches please Ma'am?"

"N12345 descend altitude 36,000 inches QNH 1022"