altimeter subscale setting

I pity the Americans flying over in the UK. Be prepared to have lots of male laughter transmitted over the radio, if you ask “Can you give it to me in inches?” 🙂

😀 😀 Says it al really

I pity the Americans flying over in the UK. Be prepared to have lots of male laughter transmitted over the radio, if you ask “Can you give it to me in inches?” 🙂

Well I got a friend to download a table off the internet, cut and paste into Word, print, fold, and store in flight suit knee pocket. Easy huh? No!

I had just spent two days flying 1200 miles to Oskosh; I was solo and about to fly into the world’s busiest airport with just one pair of eyes. The pressure setting was retrieved from the ATIS; ready reckoner was consulted, but the pressure setting on this huge list of possibilities did not exist. I re-read it, turned it over, and back and re-read it. I was even tempted to turn it upside down to find out where the number was that I wanted. It had worked all the way there and just failed at the point where it was critical for survival to be as accurate as possible.

I don’t know how many aircraft they land per second at Oshkosh, but I have seen them land three aircraft simultaneously and repeatedly and there are two runways in use at one time, as well as a separate ultralight runway and the vintage joy rides operating off of the grass strip; plus the helicopter.

I was very tempted to apply the formula Moggy C suggested, but I could only remember the conversion rate to four decimal places and I did not think that would be accurate enough! I lie. Maybe it was because I had one hand on the throttle, one on the control column, one on the map, one on the radio to retune it to the next frequency, one on the transponder and I only have five hands when I’m flying.

It was not possible to ask for a conversion as you enter Oshkosh without transmitting.

Moral of the story: If you friend is going to cut and paste a table that is may rows deep and several columns wide, make sure they copy more than just the top half of the table.

Dai — we do have ‘regional pressure settings’ — which I guess are similar to area QNHes. An RPS is the lowest forecast MSL pressure over the area in question for the hour.

Janie — I’d probably ask them for the pressure in millibars (or hPa, they’re the same after all), wait a moment, get the radio equivalent of shrugged shoulders and then the millibar reading.

Some altimeters have two scales reading inches and millibars.

Just a question. Do you use Area QNH in the UK?
Dai

30.19 x 33.865 = 1022

1022 / 33.865 = 30.19

Common denominator here is the 33.865, (1013.25 / 29.92) a simple spin on the whizzwheel can work it out for you, of course, we can all do this without putting our heads inside the cockpit for more than 10 seconds can’t we? 😉

Good question Janie,

I’d have to shrug my shoulders, say “AyeDunno?”, and ask in my best foreign accent “Que? Me no know no eenches, me only no Keeelopasquels Signor. Plees help Merci”

Failing that, I’d just look at the handy table that I always take flying with me.

Moggy :p

Stapleford is 185′ above sea level. They only work on QNH only and have a circuit height of 1200′.

So wiseguys, let’s say you are flying in the U.S. in an aircraft which has an altimeter measuring hectopascals and you are requested to set altimeter setting 3019. How do you convert it?

I think Moggy is making a point that if you fly down to the MEF, you just might hit something. Whereas if you fly down to MSA you’ve still got a tad over 1000ft of ‘air conditioning’. A subtle but big difference.

Praps not but it qualifies as a useful hint!

That is why there are big numbers printed on the maps that give minium safe altitude in that area and why that is stressed to you during flying training.

[pedant mode] What maps would these be?

UK CAA charts have an MEF figure printed large, the legend for which clearly states in big friendly capital letters

NB THIS IS NOT A SAFETY ALTITUDE

[/pedant mode]

Moggy 🙂

Oh well, since no one has bitten on my question here is the answer.

If you are flying and you are being blown to the right then what is happening to your height above level ground?

Buys Ballots law states that if you are in the northern hemisphere and you have the wind coming from behind, the low pressure area is to your left. Therefore if you rotate that through 90 degress and have the wind blowing from your left, and therefore making you drift right, the low pressure area is in front of you. Flying into a low pressure area will mean that by keeping a constant height according to your altimeter you will, in fact, be descending over level ground.

Great stuff Melvyn, we agree on all points. I was just wording it a little different.

Careful Melvyn, I sense some maths errors here:

“you might have an airfield at 3360ft and the circuit height is 3860, ie 800 feet above ground.”

Yup, simple brain out of gear but I was trying to illustrate a system, not get an “O” level so I claim immunity from mockery.
[QUOTE]
If you set QFE, the altimeter should read fairly close to zero (I use within 50ft). Keep in mind height differences around the airfield, and the time of the reading. This reading will then give you an approximate height above that one airfield.[QUOTE]
QFE is used to give a standard circuit height above a known point on the airfield (usually, for obvious reasons, the tower) and will allow you to position for the runway at known heights above ground level and be equal to other traffic. 50ft in that instance can be a big margin.

If you set QNH, the altimeter will give you your height above the sea level. It is up to you to do the maths, and calculate circuit heights etc.

Exactly the point I was making as this is what usually happens in the US. I may have made a mistake in the maths but this is the priniciple I was demonstrating. The point being that if you are gong to fly in the US get used to flying QNH alone (and don’t make maths errors)

‘Standard – 1013.25 millibars, or 29.92 inches’ is set to provide exactly that. A standard height so that traffic over the coutry can accurately gauge their height with all other airborne traffic. It still won’t tell you your height above terrain, or over the sea. MSA’s are helpful for this though.

If you wander away from the airfield (especially overflying terrain) then there is no real way of accurately ascertaining your height above the ground with the use of the altimeter alone. You need a radar altimeter for that – seldom seen in GA aircraft.

One could be very picky here and point out that Standard does not give you a height but a flight level, QNH give you altitude and QFE gives you height. It is also a basic that nothing accurately gives you a height above the ground in a GA aircraft. That is why there are big numbers printed on the maps that give minium safe altitude in that area and why that is stressed to you during flying training.

So, you are being blown right, what is happening to your height?

Careful Melvyn, I sense some maths errors here:

“you might have an airfield at 3360ft and the circuit height is 3860, ie 800 feet above ground.”

In fact, in your example, the supposed circuit height is only 500ft. This serves as a great reminder to work out your heights before you even get in your aeroplane, in a ‘no stress’ situation.

If you set QFE, the altimeter should read fairly close to zero (I use within 50ft). Keep in mind height differences around the airfield, and the time of the reading. This reading will then give you an approximate height above that one airfield.

If you set QNH, the altimeter will give you your height above the sea level. It is up to you to do the maths, and calculate circuit heights etc.

‘Standard – 1013.25 millibars, or 29.92 inches’ is set to provide exactly that. A standard height so that traffic over the coutry can accurately gauge their height with all other airborne traffic. It still won’t tell you your height above terrain, or over the sea. MSA’s are helpful for this though.

If you wander away from the airfield (especially overflying terrain) then there is no real way of accurately ascertaining your height above the ground with the use of the altimeter alone. You need a radar altimeter for that – seldom seen in GA aircraft.

It is not as widley used in the US. Airfield elevations are given and so is the circuit height so, for example, you might have an airfield at 3360ft and the circuit height is 3860, ie 800 feet above ground. The only thing to watch there is that you have the regional pressure setting for the area you are in. A dropping pressure can catch you out! The other day the local pressure reached 1040 and the following day was down to 1013. That is an 810 foot difference!

So, you are flying along at a level altitude of 1000ft on you QNH set when you took off. (Let’s assume we start at Popham which is 550 feet up, you are non radio and so you set your QNH so that your altimeter reads 550 at ground level at the airfield.) You then set out and find that you are being blown to the right. What is happening to your height above the level ground over which you are flying?

For me as a complete ignoramus without any knowledge of piloting, the following is an interesting site which helps to explain a few things including QFE:

http://www.john-a-harper.com/flying/england.htm

I had always thought that you set for zero when on the ground before departure, but now I realise that the airfield might be a few thousand feet above sea level!

Thanks for you help. I will bring this up with my instructor the next time I fly.
Nice to get a second opinion.

Thanks again

Paul, let us know what the upshot was, we’d like to know if the second opinion was correct or not.

Thanks for you help. I will bring this up with my instructor the next time I fly.
Nice to get a second opinion.

Thanks again