Stupid question – altimeters.

[Steve6443]

..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

I remember trying to measure the speed of light as a youth. I stood with a friend 1 metre apart (give or take an inch or seven - as you can see, highly experimental stuff for an 8 year old). Then my friend held a torch up and on the command of "GO" he switched the torch on whilst I started the stop watch. Once I saw the light, I then stopped the stop watch and made my calculations.....

....as you can guess, the results were wildly inaccurate ;-)

[TheiC]

Steve,


That's because you didn't measure and factor in the speed of sound, first; this skewed your result.


This is how:


Explain the following to your friend, then do it:

• Blindfold your friend
• Stand apart as mentioned above
• You shout 'Go!' and start the stopwatch
• When your friend hears you shout 'Go!', he shouts 'Go!' back
• Note: he mustn't look at you and judge this by seeing your lips move, which is why he's blindfolded - otherwise you'll be going off at half cock with the speed of light experiment
• When you hear your friend shout 'Go!', stop the stopwatch
• Halve the time measured (because the sound went there and back), then do distance over time to get the speed of sound

Factor that into your speed of light experiment, and the results should be much improved.

[Steve6443]

That's because you didn't measure and factor in the speed of sound, first; this skewed your result.

TheiC: no, I think what skewed my results was, apart from the time required for the filament to heat up and start emitting light, the fact that my friend was holding the torch out in front of himself so the distance between my eyes and the torch was significantly less than a metre away...

However, to my parent's ever-lasting torment, I think we were responsible for many a set of Ever Ready size D batteries being wasted "in the name of science" ;-)

[hegemon88]

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.

Not entirely sure about that. Last yer in Poland I flew a SP-registered C150 which had an altimeter with only the inHg subscale. A small laminated card with a conversion table was a permanent feature in the cockpit. The aeroclub has had that C150 for years.

Unless the G- world and the SP- world still wildly differ in spite of the existence of the EASA world?


/h88

[RatherBeFlying]

My favorite glider at my last club here in Canada had an altimeter with subscale in millibars -- which I suspect is the same as HPa

My procedure was: set the

1. Set field elevation
2. After release get altimeter setting from ATIS at nearby tower
3. If ATIS altimeter setting changes during flight, multiply difference by 3 (300 for the math pedants) and adjust subscale accordingly

Since most glider altimeters were not calibrated since new, it helped to check ATIS to see how far off they were

Glider pilots will say you need to judge height in the circuit by sight but it's easy to be off by a few hundred feet a few thousand feet up and subsequently begin to get concerned on downwind -- as I discovered after a frontal passage

[Heston]

...as I discovered after a frontal passage




Mmm.. its a long time since I discovered a frontal passage...

[phiggsbroadband]

Some altimeters, most likely ALL altimeters, can be re-set to the correct pressure reading by loosening the adjustment knob and re-setting, then tightening the screws up... see...


Adjusting the Kollsman Altimeter


The 'legality' of doing this yourself must be found, and agreed by engineering staff. If the altimeter is not reading the correct mB /1000ft then that is a different problem, and probably requires the back to be taken off the altimeter by an Instrument Technician. (or buy a new one.)

[Romeo Tango]

I have flown my kt/Liter/hpa aircraft in Russia and USA and managed with paper conversion tables and/or my trusty casio. One just has to be careful - in USA they even use gallons that are different to the proper kind.

[Cusco]

The Arrow I did my IR in had two altimeters, both had inches and millibars subscales (OK hecto pascals) at 3 oclock and 9 oclock.

Unfortunately the positions were reversed on the two altimeters which concentrated the mind more than somewhat.

[ShyTorque]

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.

But not recommended if you want to become an old pilot, rather than just a bold one.

[thing]

The Arrow I did my IR in had two altimeters, both had inches and millibars subscales (OK hecto pascals) at 3 oclock and 9 oclock.

Some of the crocks I fly have those, pretty handy really if you're ferrying one to the USA...

[porterhouse]

Using Metres in place of Feet to define Altitudes, Heights or Flight Levels is of course common in many parts of the world

I know of only two such places - Russia (or former USSR) and China.

[ShyTorque]

Then every chart would need reprinting to show metres, too. No thanks. Seen it in the 1980s in West Germany when the British army began playing about with our aviation charts and called them "JOGAIR". They had contours in feet but spot heights shown in metres. Totally dangerous.