Density altitude

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Density altitude

Postby Ballistic » Tue Oct 03, 2006 3:05 pm

Hey there with all the talk about density altitude, and also seeing a Cherokee 140 with 4 adult pax in it, almost hitting the deck on Sunday, this really seems to be a topic worth asking about. I do understand the concept of it but what I like to know is how do I convert it into a decision of how much weight is safe? How much runway is enough? Where is the best place to get the most accurate QNH from? ………………. Altimeter? Atis ? or should I rather keep a thermometer with me ?

Which practical tips can anybody give that will help me in my descision making before every take- off .
Fuel in hangar, runway behind u and alt above u doesn’t help
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Postby RV4ker (RIP) » Tue Oct 03, 2006 3:21 pm

Check GPS/map for ground alt. Dial that in on the Alti. It will give you a ballpark figure on the QNH. GPS alt is not accurate to the metre, but it better than nothing. Oldies believe that IF IT'S LOW DON"T GO....

There is a formula to calc effects of DA. Will take a look and see if can find...
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Postby RV4ker (RIP) » Tue Oct 03, 2006 3:35 pm

4 Sale (will trade)
P166S, Jodel, hangar and other odds and sods
Radial - http://tiny.cc/eppqp
Still @ The Coves (Harties) but dream has died
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Postby RV4ker (RIP) » Tue Oct 03, 2006 3:40 pm

4 Sale (will trade)
P166S, Jodel, hangar and other odds and sods
Radial - http://tiny.cc/eppqp
Still @ The Coves (Harties) but dream has died
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Postby RV4ker (RIP) » Tue Oct 03, 2006 3:46 pm

4 Sale (will trade)
P166S, Jodel, hangar and other odds and sods
Radial - http://tiny.cc/eppqp
Still @ The Coves (Harties) but dream has died
Ballistic
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Postby Ballistic » Tue Oct 03, 2006 3:52 pm

Thanks George the GPS sounds like a good plan and easy enough to do before taking off before every flight
Fuel in hangar, runway behind u and alt above u doesn’t help
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Postby Beenthere » Tue Oct 03, 2006 8:50 pm

Good day Juan,
I will mail you a spread sheet that will help you calculate what the D/A is in any specific place but it is a broad topic.........firstly it is a calculation that uses the QNH of the specific point you are at, the altitude and outside air temperature...which gives you the "Real" altitude above sea level, now you know that an engine looses power the higher up you are because oxygen is less, also lift is less due to lower air density and this can very depending on the temperature and barometric pressure.........Added to this if you have a tail wind, lift is lower/slower.........more weight compounds the issue, grass runway makes it worse and so we can go on. But in essence all these factors need to be taken in-to account each time you take off and land...taking off on a morning at 5 deg C and returning in the PM at 35 you have a completely different set of rules...You get away with it often...until one day...all the factors are against you and you do not know it.........My name BEENTHERE! Believe me.

Many pilots have died because of this…often it is put down to engine failure because no-body lived, a perfect example is of a 172 from East London that landed at Rand a few years back, took of later that PM with the same load and more fuel, Pilot had never taken off at 5200’ and went farming close to the “silver ball” we can go on and on……….An excellent magazine is Gary Wiblin’s Aviation & Safety, in Edition 18 there is an article on “overloading” and what the consequences are and even real examples are given……..The truth is, these pilots more than likely did this often and got away with it till all the factors turned against them…..In closing, D/A in its self can only give you a basis to work on, using all the combined factors is what is required.
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Postby DarkHelmet » Wed Oct 04, 2006 12:40 am

This is truly a scary "phenomenon" which, if I was not aware of it, would have bit me in the behind 2 weekends ago. Short field, into the wind, downhill take-off. I knew the limitations of my aerie and emptied her out, removed the panier bags to save on weight. Got airborne well before halfway down the runway but it still felt as if the fence brushed against my butt cheeks.
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Postby Ballistic » Wed Oct 04, 2006 9:37 am

Thanks Beenthere, never thought about the grass runway factor, I usually go for grass because its is more forgiving, but will def use tar for takeoff from now on. :shock: :shock:
Fuel in hangar, runway behind u and alt above u doesn’t help
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Postby Morph » Wed Oct 04, 2006 10:04 am

In short the Density Altitude increases by about 120feet per degree C the temp is above the standard temperature for your field elevation. Remember air cools at about 2 deg C per 1000ft so the standard temp for your field would be 15-(5200*2/1000) = 4.6deg C. So if you are at 5200ft and the temp is 30deg C, the DA will be about (30-(15-(5200*2/1000))X 120 = 3048 plus 5200 = 8248feet. Your engine/thrust performance of the motor is reduced by about 6% per 1000ft. So your plane will perform at (100%-49.2%)=50.8% of the performance at sea level and your performance at field level will be reduced by 20%

Your true airspeed increases by about 2% per 1000 foot so in this case the true airspeed = 3048/1000X2 = 6% higher.

How does this affect your flying.

On takeoff your engine performance and thrust are reduced. The ASI and the wings of the plane are experiencing thinner, less dense air. By the time the ASI has reached your rotation at say 50mph, you will infact have to be travelling faster. This requirement for a higher true speed will take longer in time and distance to achieve.

In flight you will require more power to fly at the same indicated speed as on a cool morning. This will use more fuel. Additionally your rate of climb will be reduced. Carrying capacity will be reduced as well as the plane's engine is running with reduced power and thrust

On landing even though your IAS seems the same, in fact you are flying relatively faster and therefore will require more runway to land and stop
Last edited by Morph on Sat Oct 07, 2006 9:43 pm, edited 2 times in total.
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Postby Ballistic » Wed Oct 04, 2006 11:09 am

Thanks morph just one question on engine performance though, it is said the my 914 turbo does have the same performance as what it would have at sea level, why ? can anyone please explain
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Postby DieselFan » Wed Oct 04, 2006 5:04 pm

I would think it best described as follows a one liner

A turbo creates an artificial atmosphere for your engine. - could be read "ideal"

Technically, a turbo is a compressor and injects air into your engine - this is a good thing. Lets say your engine needs 3 litres of air per second at 5000rpm. A turbo will supply that and the higher you go the faster the turbo will have to turn to maintain that flowrate. Typically turbos loose 1% per 3000 feet. But some can be as low as 0.2%

In the end a properly configured turbo'd engine will last longer as the operating conditions are more stable and therefore made to smaller tolerances. Another contentious issue is that a Turbo'd engine is better for the gearbox due to the flatter torque nature.

The bad reports with turbos are mostly from overboosting and maintenance. There's a saying "Turbos don't die they're killed".

On modern CPU controlled engines, when a turbo breaks the engine runs as if no turbo is fitted, however on older, less sophisticated engines it would run slightly worse than a non turbo'd version.
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Postby Thunderboy » Fri Oct 06, 2006 5:02 pm

okay these formulas are pretty big for me especially when you out on an airfield somewhere .
Is there not a rule of thumb say if we use our altimeter to determine the QNH as to when we should rather not fly or give ourselves an extra 5 or so mph before we pull back on the stick :?:
Runway lenghths are normally more than ample in lenghth for most of us so its just a matter of using them correctly :?:
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Postby Fairy Flycatcher » Fri Oct 06, 2006 5:27 pm

QNH has a little to do with it, but the biggest factor is heat and don't forget relative humidity :!: :!:

You don't have the humid days on the reef that we have down here, but it still has quite an impact.

I did density altitude calculations for our airfield ( which is at 260' ASL), and on a typical day with a QNH of 1020, the density altitude can vary from 866' ( at 20C with 40% humidity) to a whopping 3131' (with a temperature of 35C and 100% humidity - February :evil: ).

Both these calculations are at a QNH of 1020, so don't rely on your altimeter to give you the density altitude. That's the whole point! People get caught with their pants down because they don't factor in temperature and humidity.

At a runway elevation of 5200', with a QNH of 1025, your density altitude will be about 8020' when the temperature is 32C, and humidity 70%,

But at that same airfield, with the same QNH of 1025, with a temperature of 10C, and a relative humidity of 40% your denisty altitude will be about 5550'

Have a look at http://wahiduddin.net/calc/calc_da_em.htm for easy calculations. RV4ker has already put the link, just a reminder
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Postby Fairy Flycatcher » Fri Oct 06, 2006 6:01 pm

To explain it in practical terms. You take off at your airfield at 6:30 on a crisp JHB morning. For a flip with a friend. Your QNH reads 1020, temperature is 15C which gives you the elevation of your runway, and you get off in less than 3/4 of the runway. Density altitude is about 6300'

That afternoon, at about 15:00, you go to the airfield, and have another friend to take for a flight. The QNH is still 1020, and you think great!, more than enough runway. But now the temperature has increased to 30C, and even if the relative humidity is now perhaps lower in the afternoon, your density altitude is now 8070! You eat up the whole runway and see your arse!

This is Density altitude.

As the air heats up, it expands. There are even visible bubbles being formed into the Tropopause.

Each cubic meter of air then weighs less, but because of the extra heat, the pressure remains the same, as the particles are now in a more "excited" state.

Your peto-tube will register airspeed based on how many particles flow through it, and does not give a reliable indication of your speed on the runway before take-off. You have to actually go faster, the higher your density altitude, as your aircraft lifts off after displacing a certain weight of air (not volume!).

Your engine needs to burn a certain amount of oxygen, but as the air gets less dense (still the same pressure), there is less oxygen avialable per cubic liter of air. So your engine performance is less, but your aircraft needs to go faster.... you see the problem :!: :?:
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