View Full Version : Rue of thumb for density altitude?


Aerobatic Flyer
23rd Jul 2003, 22:22
Does anyone know if there is an easy rule of thumb for calculating approximate density altitude?

Thanks.



FlyingForFun
23rd Jul 2003, 23:31
Nope. Sorry.

But can't help wonderying why you'd want one. The only time I'm ever interested in density altitude is to read some performance figures off a graph. And all the graphs I've ever seen don't actually take density altitude as an input, they take pressure altitude and temperature. So I've never needed to actually know the density altitude, either by calculating it exactly or by rule of thumb. :confused:

FFF
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AC-DC
23rd Jul 2003, 23:39
FFF
When you want to take off in a very hot day from a short runway up in the mountains you want to know your density alt.

FlyingForFun
23rd Jul 2003, 23:48
AC-DC - no.

When I want to take off on a very hot day from a short runway up in the mountains, I use the aircraft's performance graphs to find out if it's possible or not. To use the graph, I need to know the pressure altitude, and the temperature - like I said in my first post. At no point do I (explicitly) calculate the density altitude.

FFF
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SuperOwl
23rd Jul 2003, 23:59
Like the above posts point out it is not something that you really will need to know all that often. But if you're curious anyway I'll show you the method I got from Trevor Thom's book 3. It's not that easy at first but with practice becomes second nature .

I did actually use it once on a really hot day at Netherthorpe with its short grass runways but I could have like those posting above just looked at the graph for the plane I would be flying in. However, my instructor believes that knowing about something is one thing, but understanding what you know is another so he made me learn it.

Perhaps you already know this method but just in case you don't....


For example, Airfield elevation 1000ft, QNH is 980. Temp 8C.

Firstly you need to work out your pressure altitude.

1000 + [(1013-980) x 30] (30 being no. of feet per millibar)
1000 + [33 x 30]
1000 + 990 = 1990 ft Pressure Altitude.

Now work out ISA Temperature at this Pressure Altitude.

ISA Temp@<hidden> MSL - (Temp change per 1000 ft x Pres. Alt in1000's ft.

15 - (2 x 2) I have rounded 1990 ft to 2000ft for simplicity
15 - 4
Which makes ISA at this Pressure Altitude 11 degrees Celsius.


Now we take the actual Temperature away from ISA for the calculated Pressure Altitude;

11 - 8 = 3 or ISA +3.

Now we multiply the deviation by 120,

120 x 3 = 360.

Next, add this value to the pressure altitude;

1990 + 360 = 2350

So now we have a Density Altitude of 2350 ft.


To sum up this method;

Density Altitude = Pressure Altitude + (ISA deviation x120)

If you have already tried this method I apologise. But it is the one that after practicing time and time again that I can do in my head. Its in Book 3 of the Trevor Thom manuals on page 129 although I have changed the example slightly in case of copyright issues.

Again, this method is harder that it looks but with practice becomes a rule of thumb. Hope this helps.:ok:

BlueRobin
24th Jul 2003, 00:02
I am not sure if I am correct but i think that you have to add to your pressure altitude 107' (or so) for each 1C above standard temp. or in other words 1000' for 10C

drauk
24th Jul 2003, 00:51
Superowl is right but makes a five course meal out of explaining it! Add or subtract 120' for each 1deg difference from standard temperature.

SuperOwl
24th Jul 2003, 01:02
I must admit, it was long winded. I nearly fell asleep whilst typing it out.

In my defence though, you should work out pressure altitude before you add any "ISA deviation x whatever" otherwise you may be hundreds of feet out.

Aerobatic Flyer
24th Jul 2003, 01:05
Super Owl

Density Altitude = Pressure Altitude + (ISA deviation x120)

Just the job. Thanks! I probably did come across something like that a few years back, but if I did I'd forgotten it!

FFF

I wanted it for interest, mostly! Interest brought about by flying in high terrain in an unusually hot summer. Until quite recently, most of my mountain flying had been done in the autumn or winter. Recently, we've often had temperatures which are ISA+20C or greater. It was starting to become hard work to fly over passes that used to be easy. Got me wondering about what the density altitude might be on those hot days.

From calculating it "properly" it turned out on some days that the density altitude at 10,000ft was over 13,000ft - at which altitude the non-turbocharged engine I fly behind doesn't want to climb any more!

I don't check the graphs before takeoff, because the runways I use are long enough and sloping. I'm not brave enough to try using the shorter sloping ones on a hot day, 'cos the turbulence is so bad that I'd probably miss the runway altogether... :rolleyes:

Circuit Basher
25th Jul 2003, 15:05
Super Owl

Sorry for delayed reply - was not in easy reach of PC yesterday, due to business trip to 'da big smoke' yesterday. I'm probably being slower than a tortoise on a wet unmown grass uphill surface (now, what's the % age margin to add for that?? ;) ), but looking at your post
For example, Airfield elevation 1000ft, QNH is 990. Temp 8C.

Firstly you need to work out your pressure altitude.

1000 + [(1013-980) x 30] (30 being no. of feet per millibar)
, I didn't understand where the '980' came from. Was this just a typo and you meant 990, in which case the Pressure Altitude is 1690 ft.

As I say, I'm probably barking up the incorrect arbour, but am willing to face a jury of my peers! http://www.click-smilie.de/sammlung/waffen/violent-smiley-021.gif http://www.click-smilie.de/sammlung/teufel/devil-smiley-029.gif http://www.click-smilie.de/sammlung/waffen/violent-smiley-078.gif

PhilD
25th Jul 2003, 15:19
SuperOwl

Am I being really dense (no pun intended!), or did you get the temperature correction wrong in your example?

ISA temp at pressure altitude 2000ft = 11C
Actual temp (in your example) = 8C

Therefore actual temp is ISA -3 (not ISA +3)

If so, the correction should be subtracted from pressure altitude, giving:

Density altitude = 1990 - 360 = 1630

Either way it's alarmingly easy to make mistakes with this stuff and end up with an answer that is miles out....

SuperOwl
25th Jul 2003, 17:25
Circuit Basher, Cheers for pointing out what was in fact a typing error, now corrected.

Phil D, it appears that in my drunken state at the time of posting my so called answer I did indeed add when I should have taken away.

Thanks to you both for pointing these errors out. I'll be more careful next time.

All the best.

buzzc152
25th Jul 2003, 18:25
Ehhh ???? Surely just get your whiz wheel out. It takes a second.
CRP-1 does density alt doesn't it ?

Bluebeard777
28th Jul 2003, 16:32
I have a small (4k) spreadsheet for my Palm PDA that I made to calculate density altitude, can copy to anyone interested.