Flying Quiz Question

[NiqueNaque]

Ted,I don't really know how this works but I'll try bullsh1tting my way through:The atmospherics drop 1 hPa per 30' of rise in elevation, starting from 1013 hPa at sea level. This means that at 30' above sea level, the pressure is 1012 hPa, at 60' it is 1011 hPa, at 3000' it is 1013 - 100 = 913 hPa, so at Cooma, 3106, the pressure should be 1013 - (3106/30) = 909.5 hPa on a standard day. However, the pressure of the air varies constantly - ever watched a barometer pinned to the wall at home, see it go up and down, you know your height hasn't changed but the pressure does. Let us supposed you are at sea level and the barometer drops to 900 hPa, you know there is a storm coming, but you have lost 113 hPa, theoretically you sould be 113 x 30' above sea level, or 3390'. This is because you lost pressure.Lets say it is a fine sunny day, you're out on the beach and the barometer is reading 1050 hPa, you should be 1013 - 1050 = -37 x 30 = -1110' above sea level, ha!, gotcha! -1110 is as good as 300m under water - do you leave the beach - is there a tidal wave coming?? no.QNH is pressure at altitude, so if you are at sea level, you set your altimeter for 0' on the dial, that gives you the QNH at sea level, on a standard day it is 1013. You take off and fly to perth, on approach they tell you the QNH is 990, so their pressure on the ground is 990, so you dial your altimeter around so the pressure reading is 990, this corrects your altimeter to read the correct height for the area at the time, this will move your altimeter hands when you do it.So you're sitting on the tarmac at Cooma, and you've set your altimeter for 3106', you look at the QNH and it reads..., ooo, have to think about this one.. shouldn't it be 1000?ASRA manual - "QNH - With this setting the altimeter will read the height above mean sea level. An aircraft at rest on the aerodrome reference point will show the aerodrome elevation on the altimeter."So at Cooma you should be able to dial in 1000 and the altimeter will read 3106' ... is this right?Sea level is 1013 hPa pressure, or one atmosphere. Cooma should be at 1013 - 103.5 = 909.5 hPa, normally. If QNH is 1000, then Cooma should be 1000 - 103.5 = 896 hPa, does this fit with the ASRA manual?But, the QNH reading at Cooma is 1000, so, 1013 - 1000 = 13 hPa loss, so 13 x 30 = 390', so its pressure altitude is 390' if the QNH is 1000 at Cooma Reference. Must have been a high pressure cell lingering over the site that day...Now I'm confused Ted,go maate.. you psyco mind messer..Go on, set me straight..wait,..Cooma should be 909.5, but it is 1000, so it gained 90.5 hPa, or lost 90.5 x 30' = 2715' in elevation, but it should be 3106', so its pressure altitude is 3106-2715 = 391' ??Maybe I'll wait for an answer..Cheers,Nick.Right Ted.. you have a barometer on the wall and your elevation is 450', the barometer reads 990, what do you advise to be the QNH?

[NiqueNaque]

Birdman,Kangaroos in the way on the road.Evel Keneivel eagles in the air..Nick.

[asra]

Well birdyI'm purty shore that means stay tha ell outa me airspace cuzz I aint lookin fer nuttin but dem cattles.[)] Am I close?cheers SonnyIntelligence is not a privilege,it is a gift and should be used for the good of mankind.

[asra]

Nick, Good try. However....The rule of thumb is as folows:"When QNH is greater than ISA MSL pressure of 1013hPa, then pressure altitude is less than elevation. When QNH is less than ISA MSL pressure of 1013hPa, then pressure altitude is greater than elevation." Keep in mind the fact that up to 5000 ft elevation, pressure falls by 30 hPa per thousand ft. The ISA MSL unit is used so that flight levels remain fairly constant for everyone. So if you didn't have access to an altimeter, you could calculate your pressure height by the following;Method:= 1013 - 1000 (Difference between ISA MSL and QNH)= 13= 13 X 30 ft/hPa (Difference in pressure altitude)= 390 ft= 3106 ft + 390ft ( Cooma elevation and QNH deviation)= 3496 ft (pressure altitude)As pressure falls below 1013 your performance will start to suffer.And to prove the theory try a QNH of 1018= 1013 - 1018= -5= -5 x 30 ft/hPa=-150 ft= 3106 ft + -150 ft= 2956 ftSo you can see that any QNH greater than 1013 will give you better performance.For those of you who have access to an altimeter, you can work out the pressure height by setting the scale to the aerodrome QNH and read off the elevation.In the very last section of your post you wanted to know what the QNH would be... 990hPa. However I believe you were chasing the new elevation right? So with elevation 450 ft and QNH is 990.1013-990 = 2323 x 30 ft/hPa = 690 ft450 ft + 690 ft = 1140 ft (pressure height)Ted

[asra]

Nick, Just say you were talking to a couple of avaitors over a beer one day and this guy said "I flew over you house at 500 ft the other day mate, and the altimeter was reading 2000 ft." You know your known elevation is 450 ft, what was the QNH?Solution;2000 ft - (450 + 500) ft = 1050 ft1050/ 30ft/hPa = 351013 - 35 = 978 hPa (QNH)Ted

[NiqueNaque]

Ted,What I'm trying to get at is: (I know.. ^*%&%&^ dumb and dumber here... due to brain malfunction, ie, thinking too mcuh.)You fly to a destination and in between the barometric pressure changes at ground level, but the elevation obviously does not, eg, you fly from preth to sydeny. I know it is impossible, but let us say you try an instrument approach based on your altimeter (airport fogged in). If the ground level is at 150' at take off - preth and the barometer was 1050 and sydeny is 100' at 990 hPa - landing, what does that mean we should set the QNH at when we get to sydeny regardless of our flight altitude to make sure that when we get to ground level the altimeter is reading the established 100'?Cheers,Nick.

[asra]

Nick, I suspect you are confusing heights with elevations. Aviation bodies throughout the world settled on the ISA standard to overcome this very issue. Irregardless of the height of any physical location, pilots need to operate within uniform flight levels, (elevations). From 0 to 179 degrees at odd thousands plus 500, and 180 to 359 degrees at even thousands plus 500 for all VFR flights up to 10,000 ft. Between 10,000 and 11,000 ft is the transition layer, and above that they are referred to as flight levelsThe way to manage all this is to set a uniform flight levels throughout the world. The International Standard Atmosphere (1013 hPa) at is for intents and purposes is an arbitary figure but is universally recognised, and is used in the setting of altimeters. In Australia, all operations below 10,000ft above mean sea level (AMSL) use mean sea level as the reference datum and so the MSL pressure is always set on the subscale. This pressure setting is referred to as QNH.With QNH set, the datum is mean sea level - for the atmosperic pressure that exists at the time. Remember that pressure changes with weather patterns too. Height above mean sea level is called "altitude", whereas height above ground is called "height".The advantage of flying with QNH set is that we can immediately use the altimeter reading to determine our vertical clearance above the ground, radio masts, obstructions, aerodromes etc, because these are all marked on aeronautical charts as "elevations", i.e. height above mean sea level. We also know that all other aircraft in the area are using the same setting. (Well they should be)For local operations, the local QNH of the aerodrome is set on the subscale prior to takeoff. The altimeter should then show close to the aerodrome elevation. At controlled aerodromes, local QNH is available from the automatic terminal information service (ATIS) or from the control tower. At uncontrolled aerodromes, you simply wind the altimeter until it indicates aerodrome elevation. The subscale will then read the local QNHTed

[asra]

Nick, In one of your posts you wrote "So at Cooma you should be able to dial in 1000 and the altimeter will read 3106' ... is this right?"If at the time, you knew the local QNH was 1000 then you'll be very close to the aerodrome elevation. It would be far better if you dialed in the elevation, the subscale will tell you the actual QNH at that time.Ted

[asra]

These interesting questions are basic to GA trained pilots. I am pleased that Ted has raised them. All pilots should never stop learning.I wonder how many Non GA trained gyro pilots thought all of this interesting enough to want to learn more about it all without actually seeking to train or fly GA aircraft ?Good work Ted.Robert DunnMackay. Qld.Growing old is good while it lasts.

[asra]

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[NiqueNaque]

quote:Originally posted by Ted EgglestonNick, In one of your posts you wrote "So at Cooma you should be able to dial in 1000 and the altimeter will read 3106' ... is this right?"If at the time, you knew the local QNH was 1000 then you'll be very close to the aerodrome elevation. It would be far better if you dialed in the elevation, the subscale will tell you the actual QNH at that time.TedThank you.This is what I was refering to, so that means the ATIS or tower has an altimeter fixed/set to the site elevation, and they read this and advise this as the site QNH. To contrast, if they had a *barometer*, without the elevation they would have to make adjustments to derive the QNH, because it cannot be read directly off a *barometer*, due to the absence of altitude adjustment on same.So, if your elevation was 100' and the *barometric* pressure was 1000 hPa, then you would have to adjust this to derive the QNH, ie, 100' / 30' =~ +3 hPa offset from scale, so the site QNH is 1003 hPa, that is what you would set your altimeter to.. The subscale of the altimeter is a means of providing the 'offset' at the site, in the example I've just quoted the altimeter is a barometer and you have dialed in a 3 hPa offset so the altimeter pointer is moved to suit.Cheers,Nick.