Temperature and pressure height calculations.

[djpacro]

When the old system of Flight Manuals was in place, as opposed to the current system of POH's - there was more safety margin built into calculating performance. The FM was factored by 15% on the manufacturers numbers - and that saved many a skin!

That requirement for the 15% margin on manufacturer's data is still in CASA's rules. Up to the operator to add it. A lot of old aeroplanes around with no manufacturers data to help the pilot. And at least one new airplane.

 

 

[motzartmerv]

The CFI of a school nearby recently was returning to the field at night with 2 others on board a 172. Apparently he took over in the fare to demonstrate how the plane could hang on the prop in ground effect. The guy in the back could see that they were climbing but the chief didn't realize. It climbed out of ground effect and stalled. Wrecked the plane but no injuries were sustained. Thank heavens.

 

 

[facthunter]

The higher you get in Density Altitude terms the less weight you can lift and this can be very marked. The concept of being overloaded as one normally considers it, goes out the window. You can easily be overloaded at 100 kgs below where you would have thought there is a problem. Balance is not affected but may become more critical as the aircrafts margin of performance is reduced. I can't recall anybody dealing with this subject properly in my lifetime, in normal basic flying training.

 

. There are commercial aerodromes at around 10,000' in some places of the world, and those who operate there use special procedures and sometimes have tbings like nosewheel brakes fitted because the lift-off speeds are so fast due to the flap settings used and the TAS being much higher than indicated, to cover the rejected take-off situation.

 

There aren't many high altitude aerodromes in australia, but some are high enough and can get hot enough to give you a

 

real problem . Some of the planes don't have vast reserves of performance. Nev

 

 

[Sapphire]

Probably of no surprise, while flying commercially I had to break the rules consistantly in minor and major ways. Was called on short notice to fly a group of three in a Mooney. The plane was loaded and fully fueled when I got there-with baggage piled high to the ceiling. Air temp was approaching 40c and used most of the 3300 foot runway. Other issues involved landing on private strips that wouldn't qualify as a driveway.

 

 

[motzartmerv]

Is a shame but seems to be all too common. The day VFR syllabus hammers the rules and regs, tests are in-depth, kdr has to be ticked off, testing standards HAVE to be met, and then once licensed, your EXPECTED to break all the rules you were FORCED to learn..

 

 

[kaz3g]

That requirement for the 15% margin on manufacturer's data is still in CASA's rules. Up to the operator to add it. A lot of old aeroplanes around with no manufacturers data to help the pilot. And at least one new airplane.

No manual or POH for the Auster! I did manage to score a photocopy of a "manual" put together by Royal Newcastle many years ago but it was for a different model and engine.

 

I do have a very simple weight and balance table but, even though it lets me put 80 kilos in the back with full fuel, I simply don't use the rear seats for pax.

 

I took Metalman for a ride in the front a while ago and he is not a little guy... very glad of the extra 5 knots I added to my short final speed!!!

 

kaz

 

 

[Guest Howard Hughes]

Some aircraft are able to take off and land with a reasonably high density height, but are then limited by the baulked approach, C172 RG springs to mind!

 

 

[Mark11]

https://www.brisbanehotairballooning.com.au/faqs/exam-help/133-pressure-density-height.htmlThat there Google thing is a great idea:book:

Great link thanks... Mark

 

 

[Sapphire]

EXPECTED to break all the rules you were FORCED to learn..

 

Training institutions have the agenda to produce a safe pilot as determined by the enforceing authority. Charter organizations are there to make money and if safety gets in the way........

 

 

[facthunter]

IF you won't do it they will find someone else who will. Nothing changes. Nev

 

 

[flying dog]

Thanks all for the replies.

 

I downloaded the sheet and shall look at it.

 

I bashed one together, but it had an error in it, so I will be interested to see what Eric's looks like.

 

I didn't quite get all the maths stuff as none of the names were declared and I didn't know what/where you were starting. Alt/temp.

 

Anyway, thanks.

 

 

[flying dog]

Not to be a "ney sayer" but looking at the sheet:

 

Putting aside the default QNH, there seems to be a problem with what is going on with the numbers.

 

I have forgotten what is what for "Pressure height, density height, and all the other names".

 

But if I am at 2000 feet and the QNH is ..... what ever. I calculate where in the ISA that QNH is and that is my height.

 

Then - I am told - I get the temperature where I am and compare that to the ISA.

 

So: If it is 3 degrees cooler, that is 3 x 30 feet LOWER in the atmosphere. If it was 3 degrees WARMER, it would be higher.

 

But you have to do the QNH calc first to get the altitude then factor in the temperature.

 

Or that is how I remember it.

 

 

[flying dog]

Correction to what I said:

 

I was told that you "gain" 120 feet per degree difference between the temp and the ISA temp.

 

So if it was 3 degrees warmer it would be 3 x 120 feet.

 

I goofed when I put in the 3 x 30 (above) (edit doesn't work second time around)

 

Where did the values used in the formulas I read come?

 

 

[flying dog]

YIKES!

 

I looked at that site for the Balooning lot:

 

https://www.brisbanehotairballooning.com.au/faqs/exam-help/133-pressure-density-height.html

 

EXAMPLE 1. An aerodrome of elevation 670 ft has an Aerodrome QNH of 1020 hPa. What is its pressure height?

 

Elevation = 670 feet

 

QNH = 1020 hPa

 

Answer:

 

Pressure Height = 670 - 7 x 30 = 560 feet

 

Ok, I am not the brightest spark, and we all make mistakes.

 

Answer: Pressure Height - 670 - 7 x 30 = 560 feet.

 

Let's look at that a bit more:

 

670 - 210 = 560.

 

What did I miss at school?

 

 

[damkia]

YIKES!I looked at that site for the Balooning lot:

 

https://www.brisbanehotairballooning.com.au/faqs/exam-help/133-pressure-density-height.html

 

EXAMPLE 1. An aerodrome of elevation 670 ft has an Aerodrome QNH of 1020 hPa. What is its pressure height?

 

Elevation = 670 feet

 

QNH = 1020 hPa

 

Answer:

 

Pressure Height = 670 - 7 x 30 = 560 feet

 

Ok, I am not the brightest spark, and we all make mistakes.

 

Answer: Pressure Height - 670 - 7 x 30 = 560 feet.

 

Let's look at that a bit more:

 

670 - 210 = 560.

 

What did I miss at school?

Laughing gas in their balloon?

I just noticed that myself.

 

The theory is OK even if the maths isn't.

 

 

[flying dog]

Yeah, ok.

 

But gee it makes me worried.

 

Here I am trying to get my head around the maths so I can make a quick spreadsheet for my phone to do that sort of stuff, and being given the wrong examples is doing my head in because I am getting different/"wrong" answers compared to what they get and I am thinking I am doing something wrong.

 

I've got that bit ok, now I just need to get the temperature correction in.

 

 

[flying dog]

P.S.

 

Damkia,

 

Just love this tag:

 

Nothing says unprofessional job like wrinkles in duct tape.

 

Top marks!

 

 

[Eric McCandless]

Attached is an updated calculator showing derivation of PH & DH as taught by my instructor.

 

Density & Pressure Height Calc Rev1.xls

 

Density & Pressure Height Calc Rev1.xls

 

Density & Pressure Height Calc Rev1.xls

[flying dog]

Thanks.

 

 

[djpacro]

Now, what are you going to do with that information?

 

 

[flying dog]

Honestly:

 

Dunno.

 

Probably nothing.

 

I probably will update/fix/use the sheet Eric put here.

 

I think in another life I was a cat. Always curious.

 

 

[Russ]

Was told this method many yrs ago.........seems to work

 

set alt subscale to the 1013, read the height indicated....let's say it reads 400ft, now guess local temp , let's say it's 34 C..........now deduct 15 off that temp of 34.....gives you 19, now multiply that 19 x120...that comes out as 2280......now add that indicated height of 400ft..............2280 + 400 = 2680ft, density altitude

 

 

[Russ]

it's now flying.

 

 

 

[motzartmerv]

Heres a video that clearly demonstrates what can go wrong when these sort of calc's are not done, or done incorrectly.

 

Everyone survives, but still, its not for the squeamish. You can clearly see the acft having difficulty climbing out, very high nose attitude and looks a bit choppy too.

 

(don't let the kids see)

 

https://www.youtube.com/watch?v=yDu0jYiz-v8

 

 

[flying dog]

I think that is the fool of whom I spoke at the beginning of my post.

 

This was on the news not so long ago.