Density Altitude

[Madman]

Wow...

Thank you Chris,now that was an awesome first post as Dish just said...

Please feel free to post some more awesome advise,info...I appreciate it and I am sure many does

Safe Landings
Rafiek

[cobra]

hereby an interesting calculator to calculate density altitude and Relative Horsepower available in %

http://wahiduddin.net/calc/calc_hp_dp.htm

[Gyronaut]

Great topic and valuable insights. I like the graph a lot since it applies to all craft.
One copy is going into each of my students' files today. Topic for the rest of the week is Density Altitude.
Good on ya!

[Dobbs]

Guys, this is a great post, and very importantly highlights the effects of temperature, and I agree that this is not dealt with in sufficient depth in our theory, and one reason for this, I believe is because our planes take of relatively quickly and with short runways. However they are just as vulnerable to density altitude. However, I do not think that we have fully closed the loop on this - ok so now we know that instead of 4500', our planes are going to perform at an equivalent altitude of 6500' - what does this mean in practice?

Other than being aware of this from a theoretical point of view, how does this relate to actual extra metres of runway length required etc? And without a comprehensive POH that details this information that all certified planes must have, we sit with a bit of a dilema.

Does anyone know of some sort of rule of thumb, that for example, if the density altitude increases by 20% your take off roll will increase by x % etc? I will do some internet searching, but there may well be readily available information.

The only rule of thumb which I know and which is applicable here, is that you must be at 70% of your take off speed by the time you are at 50% of the available runway length.

So, for those in the know, please share your knowledge, as I personally know of 3 people who crashed due to the effects of density altitude, and one of them was at the coast in a 582, so we are all potentially at risk due to this phenomena

[Flightstar]

This is an excellent thread,and we'll learn a lot from it.

My plane usually jumps into the air on take-off, but in this heat (33 C plus) the sluggish performance was more than noticeable, it was alarming .

Vereeniging is at 4840 ft , and the density altitude on Sunday was calculated as 8500 ft .

One needs to monitor the oiltemp- and CHT's closely since the engine is working harder than normal.

This density altitude thing convinced me to buy an ambient temp guage ,which will assist me in calculating the density altitude more accurately.

[Gyronaut]

I am going to suggest, to everyone I know that flies, to 'calibrate' their aircraft's performance practically.
By this I mean finding the perfect day, 15 deg, Low dew point, high pressure - all balmy - loading it full of fuel and inviting your biggest mate with you. Use the lines on the runway as distance markers or place markers next to the runway. Write the distance to liftoff and best-rate-of-climb speed down. Check MAP/Engine RPM constant etc. Then do the same on a hot day, high dew point, low pressure and repeat the exercise. REMEMBERING to abort if 70% of take-off airspeed has not been achieved by 50% of the runway. (Good reminder there Dobbs ol' Chap!). I'll try and do it here and let you know the results. Gimme some time.

By the way, do not forget that moisture in the atmosphere displaces air. Moisture does not assist in the generation of lift as air does. Also doesn't do much in favour of your trusty combustion engine. Do not underestimate its effect on Density Altitude and performance.

Keep it comin'

[justin.schoeman]

Guys, this is a great post, and very importantly highlights the effects of temperature, and I agree that this is not dealt with in sufficient depth in our theory, and one reason for this, I believe is because our planes take of relatively quickly and with short runways. However they are just as vulnerable to density altitude. However, I do not think that we have fully closed the loop on this - ok so now we know that instead of 4500', our planes are going to perform at an equivalent altitude of 6500' - what does this mean in practice?

Other than being aware of this from a theoretical point of view, how does this relate to actual extra metres of runway length required etc? And without a comprehensive POH that details this information that all certified planes must have, we sit with a bit of a dilema.

Does anyone know of some sort of rule of thumb, that for example, if the density altitude increases by 20% your take off roll will increase by x % etc? I will do some internet searching, but there may well be readily available information.

See this post by Tailspin earlier in the thread:
viewtopic.php?f=1&t=17099#p169119

Don't leave home without it!

The only rule of thumb which I know and which is applicable here, is that you must be at 70% of your take off speed by the time you are at 50% of the available runway length.

This one is always applicable, even at normal DA. Always know your abort point, and speed, before starting the take-off roll, or a lightly binding brake (or similar niggle) could land you in the trees...

[Low Level]

Big G's accident, and my experiences on Saturday got me thinking a lot.

There are other factors that one tends to overlook when DA comes to play.

First my experience on Saturday. As everyone in GP knows it was a moer of a hot day. We landed at Brits about 10:30. Brits taxiway is at the midpoint of the runway. As conditions were interesting, I went for the numbers at the beginning of the runway, then low level along the runway, about 2 meter above the tar, to feel conditions, before I commit for landing. Just before the midpoint, when I wanted to flare, it was rock and roll. Wind was coming from about 11:00, but the gyro was shaking violently from side to side. I had to do some serious corrections to land.

We took off at Brits round about 12:15. Dunno what the temps was, but it was close to 40. Standing on the threshold, a Cessna 182 came in for landing. I said to my pax, watch this. Coming over the threshold, it started to hang, just wouldn't go down. When they hit THAT spot, it started getting interesting. The plane was all over the place. The pilot then WANTED to land, cause he was going deep. He achieved getting the wheels to stay down on the fourth attempt, by moering it down.

The take-off was a non event, BUT, I really had to manage. I usually climb out at 75 mph. This time I was doing it at 65, cause that is all I had at full power, watching my VSI like a hawk. I was about a mile past the end of runway before I could even attemp to turn crosswind, and then VERY gently. While flying at that time, there was NOTIBLE updrafts and downdrafts going over different colours of veld, roads, factories etc.

NOW, with all this brabble, the point I want to make. With serious variations in DA, one tends to concentrate on what is going on, ON the runway. Take off roll, point of no return, etc. , BUT what is happenning when the runway ends? For instance, Rhino park has a dam after 09, used to have swamp after 27, Aeroden is grass, with a tar road just after take off on 36.

Although I was in the air using only a third of the runway, I was climbing, and speed was in the green zone, I was only managing. What if I was crossing a dam or swamp at the end of a runway at 100 ft AGL, flying through a serious pocket of high humidity. Would I still be managing?? What is happening over that tar road after take-off at Aeroden?

The wake-up call for me, after reading Grants recap of the accident, was this. One cannot relax when you are airborne, and have just enough airspeed after take off. You are still in a very vulnerable situation. Watch for signs on the ground, especially at airfields to which one is not accustomed to.

[Dobbs]

The only information I have on Big G's accident is what I have read on this forum, and more importantly, Big G's version of events, so please do not shoot me down if I am out of line

If I had to hazard a guess, I would say that although the density altitude may have been particularly high, the fact that he got airborn and was climbing out, implies that DA was not the root cause - I am inclined to believe that this accident may well be related to a dust devil that may not have yet reached the ground, hence the absence of any indication of it.

Having at least two wind socks a couple of hyndred meters apart is one way of confirming the presence of thermals and the possible formation of "dust devils", but I am not sure how accurate an indication this is of their formation

I heard of an LSA at Upington, some years ago, having a hard landing with some damage to it after penetrating what the pilot believed to be an "un seeable" dust devil on finals.

So if this was the cause / contributor to Big G's accident, I am not sure what we can do to ensure that none of us have to experience the same or worse experience

[Dish]

I agree with Dobbs. _ and also just my opinion, but, there were a few flights conducted before the accident and in fact Demon had just taken off with no problem- Im sure the "thinness' of the air i.e DA had a role in the matter, BUT, reading G's post and listening to people my opinion is iand remains it was an un seen Dust devil. In my experience normally you can see them. In this case he couldnt -

So
"So if this was the cause / contributor to Big G's accident, I am not sure what we can do to ensure that none of us have to experience the same or worse experience " (Dobbs)

I believe the answer is practice caution at all levels (obviously) .... lets be honest lads.... most trikes are tucked up in hangars with covers on well before 10.00a.m on a normal morning never mind in the middle of a heatwave.. - the later it gets the more prone we all are to thermals, DA and Big winds... we cant predict events like this, but we can make choices. Hot, humid, Low QNH, gusty conditions/?????/ remember the beer is cold and fly another day..

When I flew on sunday, the QNH was LOW (1010) but that was it. No Wind, had just rained so air was crisp, and all i really had to contend with was a C** climb rate.

I know Grant to be an incredible pilot and know that he would have considered all the angles before taking off... but thats why its called an accident??? Make the best decisions you can back yourself and be safe.... ??

[Mogas]

+1 on the Koch chart, a very useful "rule of thumb" chart to carry in the docs folder.
Also remember that the fully correct calculation of DA takes into account humidity, as mentioned earlier, ISA assumes dry air.
Whilst humidity may not affect aerodynamic performance to the extent that temperature does, it's affect on engine power output is significant.
This is due to water vapour occupying space that would otherwise allow for more oxygen in a given portion of air, thereby reducing the power output of an internal combustion engine.
So, whenever all or most of the Hs are present: HOT, HIGH, HEAVY, HUMID it's time to haul out the POH, find the sea level performance figures and use the Koch chart accordingly. The Koch chart does not factor in humidity so err on the side of caution if the runway is short and you are at MAUW.
Another thing about ISA (International Standard Atmosphere).
There is an obvious need to have a set of standard conditions to use as a yard stick, unfortunately for us in this part of the world we virtually never see ISA conditions. The makers of our engines and airframes all quote standard conditions when they write the performance manuals because this is the industry standard. Just be aware these numbers in the POH are for a nice new engine operating at ISA.
From what I have read, I also don't think this was the cause leading to Grant's crash. A good thing to come out of that unfortunate accident is to see this topic brought up again, we all need reminding of it's importance on a regular basis.

[cobra]

The following link will give you the density altitude and relative horsepower % available (current conditions and forecasts up to 7 days). The example is for Aeroden and is available for any place in south africa. Select upper winds for 3 hourly "spotgraph"

http://www.vfrplanner.co.za/?page_id=24 ... uth+Africa

[Chris Sibbald]

Factors affecting air density are: Pressure, temperature, Humidity and Height !!!

When it comes to height, we have to remember that air has weight, therefore at mean sea level, there is more air above, which will exert a greater pressure than at height. The greater the pressure, the greater the density. Now with an increase in altitude, pressure decreases, so does air density, at an average rate of 12% with an increase in altitude of 3000 ft. But temperature decreases at 2% for every 3000 ft increase in altitude, cold air contracting and therefore increasing density. So combining both the effects of altitude and temperature, density decreases at an approximate average of 10% with every 3000 ft increase in altitude.

A usefule "rule of thumb" method to obtain the density at a level 1000 ft higher is to subtract 3% of the value at its present level. This evaluates almost exactly up to the density value of 20 000 ft when using ISA

Chris