I just had to say it...

[Adrian Lewer]

the whizz wheel is such a great tool. i have spent the last week looking at it dumbfounded but have been playing with it and reading the bak and bob tail nav book and it explains a fair bit. i can now estimate time to waypoint, actual ground speed in the air, which will in turn give me my head wind reading :) . i am excited that i can now actualy do something with it :) .

 

now all i need to know is how to do the density altitude thing (it says not needed in LSA but would like to know anyway) as the wheel i have is different from the one in the book and cant figure it out....

 

anyway happy blabbing is over :)

 

 

[Guest pelorus32]

G'day Ferret,

 

anyone who tells you that you don't need DA in LSA should be made to sit beside you when you are very hot and very high and at MTOW somewhere in the Snowy Mountains on a short strip in late January.

 

You do need to know this. Don't have time right now (sneaking a break from a meeting ;)) to do this in detail but you do need to know it and its affect on performance for your a/c.

 

Regards

 

Mike

 

 

[Adrian Lewer]

hi mike,

 

believe me i do agree with you this is why i have been trying to figure out how to find the DA on the whizz wheel (which i think i found out last night at work)

 

the bob tait book said "for light aircraft, CAS and IAS can be concidered the same."

 

but when you do the math in the book (one of the practical examples, airspeed modified for slower u/light speed)

 

cruise altitude of 5000 ft (pressure altitude) , OAT +22, CAS 55 kts , which is CAS of 55 kts and TAS of 56.5 kts,

 

1.5 kts over 80 Nm is.... well 55 kts CAS over 80 Nm is 87 mins

 

56.5 kts over 80 Nm is 85 mins,

 

so you will plan an 87 minute trip and you will be looking for an airfield which is actually 2 minutes in front of you, actually i will re phrase that you will overshoot the waypoint by 2 minutes..... and be looking for a waypoint 2 minutes behind you...

 

but if you where in a faster aircraft like a jabiru you will be a few more minutes off course..

 

now, my next question to get off my chest is, when you are doing pre flight and you set your alt to 1013 qnh which is about 1430 ft AMSL at ballarat, is this the pressure height ? if so am i correct in saying that when you make these calculations you use the Pressure altitude displayed on your altimeter ? IE: circuit height at ballarat is 2430 Ft which is also the Pressure height ?

 

does pressure altitude ever change ? like one day it will be 1013 QNH and tomorrow it will be 1020 ?

 

can i set my altimeter to the QNH for a x country flight and have the correct height for all airfields i will visit ? for arguments sake from ballarat to narromine ?

 

ah too manny more questions but need to understand these first.

 

 

[Mazda]

Ferret remember that navigation is based on groundspeed, and that can be quite variable. The wind is not always as forecast and can vary enroute, certainly by more than 1.5 knots! Estimates are regularly updated in flight based on actual groundspeed.

 

Yes, the pressure does vary from day to day, and even hour to hour, and your altimeter (set to QNH) will only read pressure height if the QNH for that moment happens to be standard (which is not all that common).

 

There is a published area QNH for different times in each forecast area. You can use that forecast QNH for each area, or you can use the QNH of an airfield within 100 nm. So yes, you do change QNH enroute. Your destination could have an AWIS, ATIS or forecast QNH which you can use for your arrival.

 

Your instructor will cover all of this, as will your theory studies!

 

 

[Yenn]

We don't use 1013.2 below 10,000' unless it happens to be the area QNH. It is used in controlled airspace above the transition level for IFR flying.

 

We should be using area QNH which can be found from the radio area frequency.

 

For local flying just set the altimeter to the level at the airstrip and the area QNH will be very close to what you see in the millibar scale on the instrument.

 

The other figure sometimes used is QFE, which is the setting to get a Zero figure on the altimeter.

 

If you set 1013.2 on the altimeter. To get density altitude on the computer.

 

In the airspeed correction window, align your indicated altitude with the outside air temp and then you should be able to read density altitude in the density altitude window. This is not absolutely correct as you should be using calibrated altitude but there is little difference between indicated and calibrated, especially at the heights we fly.

 

 

[facthunter]

common reference.

 

One of the purposes of getting an altitude reading that uses an appropriate reference (used by all operators in the same airspace) is to provide separation (vertically). Eg. if you are talking to someone and you say that you are cruising at (whatever) altitude. and they tell you that they are (say) 500' above you, you are only separated by 500' IF both altimeters are set to the same pressure reference, which below 10,000' is the area QNH, which gives a height above MSL. (Mean Sea level). This (the QNH) MAY change as time progresses, Or as you move across the surface of the earth, particularly when there are strong winds which indicate the existence of a pressure gradient (the isobars are close together, on a weather map). IF the QNH is accurate, and your altimeter is accurate, it will read the aerodromes height above sea level when you touch down, consequently, if you set the aerodrome altitude on the altimeter, you should read very close to the area QNH on the altimeter subscale.

 

..... If you set the subscale to a figure where the altimeter reads zero on the ground, then the mathematics of getting your circuit heights right becomes simplified, AT THAT AERODROME. By definition this corresponds to something called the QFE. While it is used in some parts of the world ,I have never used it and have never seen it used in australia. & with a view to standardisation I would not recommend it in a training environment. There might be an argument for using it in mustering or cropspraying (anyone know?)

 

Nev...

 

 

[Guest pelorus32]

But we seem to have all skipped over Ferret's first question which was about Density Altitude. The key issue for me about Density Altitude is its effect on a/c performance.

 

When I'm not frantically working for the man I'll sit down and do a post unless someone beats me to it. Nev?

 

Regards

 

Mike

 

 

[Adrian Lewer]

now i need a melways.. i am realy lost. but having said that i have learnt alot in the last 2 weeks of having my w/wheel and my nav book.

 

as for seting the altimeter to read 0 before flight agree that i would never do it... to dangerous in my book...

 

hopefully i will get into some navigation soon...

 

Adrian

 

 

[Guest pelorus32]

Density Altitude:

 

Let's say that you are at an airstrip in the Snowy Mountains. The airstrip is at 3000' AMSL. Today is a not so good day in January. The QNH is 1002 hPa and the temperature is 35 degrees C. The strip is 400 metres long and that's OK because the aircraft you fly usually gets off in 200 metres. But you just want to be sure because your mate is coming for a fly and he's a big fellow and you'll be at MTOW and it is hot.

 

To do that you need to understand what the Density Altitude is - that's the density of the air that the aircraft will operate in and it determines aircraft performance. The International Standard Atmosphere is based on 15 degrees C at sea level with a QNH of 1013.2 hPa. We use 15 degrees and 1013 as an approximation.

 

1) First thing to do is to calculate Pressure Altitude:

 

PA = Elevation + 30*(1013-QNH)

 

PA = 3000 + 30*(11)

 

PA = 3330 feet

 

Pressure altitude varies by 30 feet per hPa change.

 

2) Next Calculate the ISA temperature for 3330 feet:

 

ISA temp = (15 degrees - ((3330/1000)*2))

 

ISA temp = (15 - 6.66)

 

ISA temp = 8.34 degrees

 

ISA temp at a rough approximation falls by 2 degrees per thousand feet pressure altitude.

 

3) Calculate temperature difference

 

Temp Diff = Ambient temp - ISA for PA

 

Temp Diff = 35 - 8.34

 

Temp Diff = 26.66 let's say 26.5 degrees

 

4) Calculate Density Altitude:

 

DA = PA + (Temp Diff * 120)

 

DA = 3330 + (26.5 * 120)

 

DA = 3330 + 3199

 

DA = 6529 feet

 

Each degree of temperature difference increases (or decreases) the DA by 120 feet.

 

So what this means is that you are at 3000 feet field elevation but your aircraft will behave as if it is at 6500 feet field elevation. Go to your POH and look up how much strip you need to get off (and indeed if you can get off) at 6500' DA.

 

In addition be aware that the IAS is impacted by the lower air density and so you will have an IAS of whatever you normally take off at but the TAS will be 13% (roughly) greater than that so you will eat more strip as you will when landing.

 

Please others chime in here.

 

Regards

 

Mike

 

 

[facthunter]

more....

 

Why? you've covered it all. Very competently too (if I may say so). The big thing to notice is the effect of the temperature. Nev..

 

 

[Mazda]

I have used QFE here - but only for aerobatic competitions operating out of one field, where the aerobatic box has defined boundaries including an upper and lower limit AGL.

 

It is not necessarily dangerous to set zero on the ground (that is what they do in the UK), but it is not standard practice here and not recommended for normal operations when other pilots will be using QNH.

 

 

[Yenn]

I thought ferret asked how to work it out with the wizz wheel, that is why I posted "In the airspeed correction window, align your indicated altitude with the outside air temp and then you should be able to read density altitude in the density altitude window" Was that what ferret wanted to know?

 

 

[Adrian Lewer]

indeed i did ask that question ian but .... having said that all the info that has been wrote here is all great.

 

for me flying around the circuit and training area is easy but the nav's is going to be some real work, but as i said i am learning heaps everyday.

 

 

[Adrian Lewer]

Mike re: your equasions above :

 

PA = Elevation + 30*(1013-QNH)

 

PA = 3000 + 30*(11)

 

PA = 3330 feet

 

i do not understand... Pressure altitude = 3000 ft + 30* (30* what is this figure)

 

 

[Guest pelorus32]

Mike re: your equasions above :PA = Elevation + 30*(1013-QNH)

 

PA = 3000 + 30*(11)

 

PA = 3330 feet

 

i do not understand... Pressure altitude = 3000 ft + 30* (30* what is this figure)

G'day Ferret,

 

OK:

 

PA = Pressure Altitude

 

Elevation [that's the height of the field and is in our example 3000']

 

"30" this is the factor that converts the change in QNH to altitude - for every hPa above or below standard the pressure altitude increases or decreases by 30 feet

 

"1013" is the ISA standard air pressure in hPa

 

QNH is the barometric pressure on the day and in our example we said it was 1002 hPa.

 

So:

 

PA = 3000 + 30*(1013-1002)

 

PA = 3000 + 30*(11)

 

PA = 3000 + 330

 

PA = 3330

 

The (1013-1002) term calculates the difference in hPa (11) between ISA and the current QNH. Multiplying it by 30 converts it to feet of pressure altitude.

 

Then you just have to add it to your field elevation to get the PA.

 

Hope that explains it, if not come back again.

 

Regards

 

Mike

 

 

[Seal]

hi mike, the bob tait book said "for light aircraft, CAS and IAS can be concidered the same."

Great explanations of the PA and DA calculations but I wondered if this definition may still be causing an issue. The Calibrated (or corrected)airspeed (CAS) is the Indicated airspeed (IAS) corrected for measurement errors, that is errors in the instrument itself or the pitot static system. So, to be pedantic you take the IAS, correct it to give CAS then apply all the other corrections to get TAS. As the man said, in practice IAS=CAS works for us.

 

Also, if you don´t feel like calculating your Pressure Altitude you can set you altimeter subscale to 1013 and it will then read PA. Stick that and the temperature into your whizz wheel and hey presto.

 

 

[Adrian Lewer]

Yep sounds Like a great plan... i will try next time we are flying... (friday) :)

 

 

[skybum]

Hey Ferret, just read your post for the first time.

 

Given OAT= 22C

 

Altititude=5000ft given QNH is set

 

CAS=55kts

 

Whizz Wheel says TAS=61kts and density alt of 7000ft

 

Make sure you read off +temps to left of 0 against the correct altitude, I think you had set -22C against 5000ft to get a TAS so close to CAS.

 

 

[FlyBye]

I think the subject as been covered fairly well but this site maybe of some use.

 

http://www.auf.asn.au/groundschool/umodule3.html#density_altitude

 

 

[Adrian Lewer]

Yep i think you are right, i used - instead of +, good pick up.... i have sat down with my CFI and had a little chat about DA amongst other things, and have a slightly clearer view on the subject. will be getting into more detail soon when the navs start.

 

 

[old man emu]

Ferret,

 

Looks like you have learned how to carry out the process required to calculate Density Altitude, but do you have an understanding of why you would need to calculate it? That's the important thing to have.

 

Imagine that you are planning to fly on a Summer's day. Air temp is 35C; QNH 1020; Airfield Altitude 557' AMSL and there's no wind. What warning flags would these figures raise in your mind?

 

Old Man Emu

 

 

[eastmeg2]

I may get bagged for this since we're not suposed to rely on our electrinic toys for information, but once or twice after take off and established in cruise I've realised I had not set the QNH and set it to make the altimeter match the altitude reading on my GPS.:hittinghead:

 

Far from ideal I know, but probably better than nothing in the absence of better information. Garmin do claim they're usually accurate to within about 30ft with the disclaimer that you should always refer to your aircraft altimeter first.

 

 

[old man emu]

Obviously no harm done, but why didn't you just grab the radio and ask what the QNH was?

 

Everyone else around you would have been flying on Area QNH, which might have been different from the barometric pressure at the precise point your aircraft was located. I've no doubt about the accuracy of your GPS. It is probably as nearly as accurate as a radio altimeter.

 

Your approach to the problem was better than no approach at all.:big_grin:

 

Old Man Emu

 

 

[Adrian Lewer]

hi, nope i have no idea of why i would need to calculate it on a day like that... my guess would be airframe/engine performance ? is this correct ?

 

 

[old man emu]

hi, nope i have no idea of why i would need to calculate it on a day like that... my guess would be airframe/engine performance ? is this correct ?

Actually your guess shows that you do in fact have an idea, and it's correct.

 

35C is a hot day. No wind to assist gaining airspeed. The effects on your aircraft's performance should be considered. Will these conditions result in a longer ground run before take off? Can your aircraft take off with full tanks and full payload?

 

What is the answer to the Density Altitude calculation for the figures I gave? Would your response to the answer be

 

a. There's no significant effect

 

b. I'll have to reduce the amount of fuel/payload

 

c. The aircraft won't get of the ground today, better track direct to the club bar.

 

Old Man Emu