Temperature and pressure height calculations.

[flying dog]

Partly in light of that FOOL who couldn't take off and crashed into tree tops with 3 other people in the plane, I am trying to remember the formula of how to calculate temperature height and density height.

 

You do temp first, then density - right?

 

What is the formula?

 

So far I have:

 

Pressure Height = Elevation + 30x(1013-QNH)

 

But that is AFTER the temp' calculation.

 

Someone?

 

 

[damkia]

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

 

That there Google thing is a great idea:book:

 

 

[djpacro]

I haven't seen that video and know nothing of that accident so no comment from me.

 

I know that density height calculations were of no earthly use to me when preparing to fly an early Model Cessna 172 one day.

 

Elevation 6200 ft. TAF for a nearby airport right now is

 

110522Z 1106/1206 31007KT P6SM VCSH SCT100 BKN140 FM120200 VRB06KT P6SM SKC

Probably something like it was that day.

The U.S. has no requirement to forecast temperatures in an aerodrome forecast .....

I knew that it was going to be warm, around 25 deg C. I was current in that airplane and had been flying from that airport for ages as I lived nearby. Only difference was I usually flew two up but that day I would have two children in the back so a little heavier. Density altitude was going to be higher than when I had last flown it. I spent quite a bit of time assessing the data in Cessna's excellent POH.

Runway length was nearly 6000 ft. Take-off was normal into the gentle breeze straight down the runway into the north, using about half the length to get to 50 ft above ground. Positive climb until about 300 ft above the ground.

 

Positive descent. Check. Full throttle, carb heat etc. Speed for best rate of climb. Descent continued.

 

 

[frank marriott]

ISA 1013 @ 15 deg

 

PH. 30/mb diff from 1013

 

DH. 120ft/deg diff from 15 deg

 

FrankM

 

 

[facthunter]

The ISA ( International Standard Atmosphere) is based on 1013.2 Kpsa/sea level and 15degrees C and a standard lapse rate of 2degrees C/ 1,000 feet

 

If you were on the ground at 5,000' ( on the altimeter) with your Kollsman scale set to 1003 and your OAT read 28 degrees C. What would your density altitude be?

 

Note. We are not taking Humidity into account for the purpose of this exercise, but high humidities DO increase density altitude, despite some assertions to the contrary. I've given some pretty easy figures so away you go.

 

Density altitude is important because it will be what height the PLANE thinks it is at as far as engine and aerodynamic performance is concerned. Nev

 

 

[djpacro]

All good but no use at all if one does not know what to do with the number or if there is no performance info provided for the airplane. (Also, some POHs simply use pressure height with tables for different temps)

 

 

[Eric McCandless]

I hope I have it right? (Edit: no I didn't, I transposed an incorrect value, hopefully correct this time)

 

+++++++++++++++++++

 

Density Height

 

15 - (PH*2/1000) = x = ISA temp at pressure height

 

OAT - x = y = ISA deviation

 

DH = PH + (y*120)

 

Pressure Height

 

1013.2 -QNH = x = pressure variance

 

x * 30 = y = pressure height diff

 

PH = y + elevation

 

+++++++++++++++

 

therefore, calc pressure height

 

1013.2 - 1003 = 10.2

 

10.2 * 30 = 306

 

306 + 5000 = 5306 = PH

 

now calc density height

 

15-(5306*2/1000) = 4.39

 

28 - 4.39 = 23.61

 

DH = 5306 + (23.61 * 120) = 8139.2 feet.

 

 

[Sapphire]

Guess the airline guys calculate density height all the time esp for take off. Ever been in a fully loaded 747 bound Sydney to Los Angeles on a really hot day? Glad I can't see the remaining bit of runway.

 

For the a/c we fly I guess you go by experience and feel. Would never take off with a new a/c from a short grass strip on a hot day with variable winds unless the previous owner was there to demonstrate it first.

 

 

[facthunter]

There are calculators that take into account ALL of the factors . A POH with height and temperature goes most of the way, but that info is not provided for any U/L that I know of..

 

They might consider the service and absolute ceiling of each aircraft at differing weights and then the density altitude would have relevence., but such a chart does not exist.

 

Without the Humidity it is not the complete picture either.... The only purpose is to work it out and if it comes to a high figure you might have some idea of how poorly your plane will be expected to perform. A gazelle with two onboard at any DA above 4,000' is a bit of waste of time. One has to know the aeroplane they are flying. I think it is worthwhile to do a few calculations to see how big the effect is. Nev

 

 

[Sapphire]

I hope I have it right? (Edit: no I didn't, I transposed an incorrect value, hopefully correct this time)+++++++++++++++++++

 

Density Height

 

15 - (PH*2/1000) = x = ISA temp at pressure height

 

OAT - x = y = ISA deviation

 

DH = PH + (y*120)

 

Pressure Height

 

1013.2 -QNH = x = pressure variance

 

x * 30 = y = pressure height diff

 

PH = y + elevation

 

+++++++++++++++

 

therefore, calc pressure height

 

1013.2 - 1003 = 10.2

 

10.2 * 30 = 306

 

306 + 5000 = 5306 = PH

 

now calc density height

 

15-(5306*2/1000) = 4.39

 

28 - 4.39 = 23.61

 

DH = 5306 + (23.61 * 120) = 8139.2 feet.

My old Kane Dead Reconing Computer comes to about the same figure. He is using 1003 now.

 

 

[facthunter]

I replied to Eric but something went amiss. I reckon hegot it out. Try it again with a new QNH 1003. You can get lower than that but the weather is getting worse. Nev

 

 

[Eric McCandless]

I have put together an excel spreadsheet calculator if anyone wants to use it (at you own risk as I am just a student not an expert).

 

Density & Pressure Height Calc.xls

 

Density & Pressure Height Calc.xls

 

Density & Pressure Height Calc.xls

[turboplanner]

So Sapphire and Facthunter how do you discharge your CASA obligations without manufacturer data, and how do they get away with it?

 

The Stinson isn't all that much removed from a Jab.

 

 

[Sapphire]

So Sapphire and Facthunter how do you discharge your CASA obligations without manufacturer data, and how do they get away with it?

 

Gulp

 

 

[Eric McCandless]

There are calculators that take into account ALL of the factors . A POH with height and temperature goes most of the way, but that info is not provided for any U/L that I know of..They might consider the service and absolute ceiling of each aircraft at differing weights and then the density altitude would have relevence., but such a chart does not exist.

Without the Humidity it is not the complete picture either.... The only purpose is to work it out and if it comes to a high figure you might have some idea of how poorly your plane will be expected to perform. A gazelle with two onboard at any DA above 4,000' is a bit of waste of time. One has to know the aeroplane they are flying. I think it is worthwhile to do a few calculations to see how big the effect is. Nev

There are no PH/DH limits shown in my J170 POH. It says the J170 has a service ceiling of 10,000 feet.

Can I assume that as long as the calculated density height (and pressure height) is less than 10,000 feet, I can take-off at MTOW?

 

On second thoughts, that I assume it would be the maximum height to maintain level flight only, insufficient to climb. So how can you calculate the impact of PH/DH for safe take-off?

 

The POH lists take-off distance 475 m, and landing distance 468 m. Would that be at sea level? If at sea level, how can I calculate the revised take-off and landing distance for different PH/DH?

 

 

[turboplanner]

This probably relates to the rapid change in RA from flying blown up Coles shopping bags over your own paddock to touring where the departure and arrival airfields can be at different altitudes.

 

 

[motzartmerv]

http://lmgtfy.com/?q=Pressure+height+calculation#

 

:augie:been waiting to use that...

 

 

[Sapphire]

The POH lists take-off distance 475 m, and landing distance 468 m. Would that be at sea level? If at sea level, how can I calculate the revised take-off and landing distance for different PH/DH?

 

I always found manufacturer take off and landing distances incomplete in their advertising hype. That would be at sea level-but who flies at sea level? I have a graph for a Varieze at different density heights giving exaggerated performance figures, but other a/c have none. By the time you do the calculations and measure the length of that unknown grass strip you are on, you'll be looking at past last light. I take with caution any published figures of landing/take off performance, weight and balance etc. Seen them not work, especially if you fly on the limits of that flight envelope.

 

 

[facthunter]

Absolute ceiling is just that. Service ceiling is when your Rate of Climb gets to 500 fpm. I would NOT assume that gave you take-off capabiltiy.

 

Turbs. I have always said, "know your aircraft". Obviously little of this is specified for RAAus, and I doubt if many with GA understand it either. Hotham and Gyra and Orange I have personal experience with and Gyra was closed I believe some time back. I recommended that happen for the organisation I flew for then. ( At least in summer conditions). RAAus planes vary so much and as has been mentioned if you believe some of the figures claimed, you should believe in the tooth fairy too. Ground effect may help you to a max height of about 2/3 wingspan.. The hardest thing to appreciate is the power reduction and prop efficiency loss at high DA's. ( on top of the wing not wanting to do much for you either... Nev

 

 

[turboplanner]

I remember one C172 crash featured in the Aviation Safety digest. Farmer flew across to the neighbours for a machinery part early in the morning, landed on a claypan.

 

They got talking, looked around the farm and he took off from the claypan much later in the day with the machinery part, one or two passengers who wanted to look around and the temperature had climbed much higher.

 

No calcs at all.

 

The 172 clearly wasn't going to lift off the clay pan, evidenced by the tracks, so he apparently decided to use the bank of the claypan as a launching ramp.

 

the wheel left the ground but OOPS! there was a phone line in his windscreen. He couldn't climb so he decided to bounce it back under the lines then yanked the stick back again but sank into the ground and killed all on board.

 

Sure stuffed that morning for the local community

 

 

[Sapphire]

I've heard of so many similar cases where the pilot loads up and thinks it's ok when the plane does climb away, then comes back down past the strip after leaving ground effect. If you have to fly really overwight, stay in ground effect till your speed builds up and then climb at a very low rate [100fpm]. If terrain doesnt allow it-dont go. Thats what I would do.

 

 

[Ryanm]

Performance calculations are something that unfortunately does't seem to be drilled into students as much as it should be. You will be on your own if you decide to depart on a day outside the performance charts of the aircraft (insurance wise anyway). Having a "feel" for the aircraft is one thing, but it might bite you one day. That video doing the rounds at the moment is a good example...

 

 

[kaz3g]

I've heard of so many similar cases where the pilot loads up and thinks it's ok when the plane does climb away, then comes back down past the strip after leaving ground effect. If you have to fly really overwight, stay in ground effect till your speed builds up and then climb at a very low rate [100fpm]. If terrain doesnt allow it-dont go. Thats what I would do.

I think the better answer is DON'T fly overweight anytime at all.

 

Kaz

 

 

[Sapphire]

He couldn't climb so he decided to bounce it back under the lines then yanked the stick back again but sank into the ground and killed all on board

 

He might have made it but was doomed as soon as he decended beneath the lines. He was already near the stall and to arrest his decent he "yanked the stick back again". Now the aircraft had to suddenly produce both more lift to arrest the downward momentum and more lift to climb away. There was no lift left to produce as the stall angle of attack was now reached and the a/c "sank into the ground and killed all on board".

 

Of course don't fly overweigh anytime but that tragedy will repeat itself again and again

 

 

[poteroo]

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! We still use the old charts where we have them.

 

Where I believe pilots underestimate their needs is mostly in lack of accurate measurement of : (1) - the real headwind, (2) - the real air temperature (3) - the real weights involved . If all of these were actually measured, on the spot, the decision to reduce weight, or not attempt the take-off, would be more realistic.

 

happy days,