Compensating for Density Altitude

azpilot

azpilot

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azpilot
I'm curious what most of you do to compensate for density altitude on those warm summer days at higher elevations. Is there much I should be doing beyond leaning and verifying my takeoff distance in the POH (with appropriate margin)? Would you have concerns taking off at max gross weight at a field with 7000' DA assuming the runway is "long enough"?
 
Even off the ground the rate of climb will be diminished. You need to make sure you are going to outclimb any terrain or obstacles.

Also note that on approach/landing the indicated airspeed will result in a higher true airspeed (and hence ground speed) so that will be a different visual sensation.
 
jordane93 said:
Run the numbers and compare it to the charts.
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What kind of margins do you typically use? If the book says you need 2000', you're probably not taking off on a 2000' strip.
 
Where I live in summer the density altitude can easily reach 10,000 ft.

I have seen planes land and not be able to get off the ground. I will suggest to them to try leaving at 5am at the coolest part of the day.

azpilot said:
Would you have concerns taking off at max gross weight at a field with 7000' DA assuming the runway is "long enough"?
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Sure, as long as I can clear the terrain in the area.
 
After you run the numbers,check the terrain . Then decide what margin you add for comfort. Also decide your abort spot before the take off run.
 
Expect a long take-off roll. Set an abort point on the runway before you go, or use a rule of thumb - I think I have read; "If I'm not at 2/3 my liftoff speed by 1/2 the runway I will abort." (someone pls correct the details on that one)

Mentally prepare for anemic climb rates.
You cannot have the same expectations at 8000' DA vs a sea level airport but many people do.
Don't be in a hurry to yank it into the air; don't climb out of ground effect in a big hurry.

Many hi DA airports have at the fbo some very good advice, suggestions. Angel Fire, NM KAXX is one example; the lounge room is full of helpful suggestions.
Same with long time pilots at these airport; lots of great advice goes unasked for.
 
azpilot said:
I'm curious what most of you do to compensate for density altitude on those warm summer days at higher elevations. Is there much I should be doing beyond leaning and verifying my takeoff distance in the POH (with appropriate margin)? Would you have concerns taking off at max gross weight at a field with 7000' DA assuming the runway is "long enough"?
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The first rule of thumb to learn is that only 90% of the book max gross is used for high density altitude airport operations. Many folks have learned the hard way when they can’t climb out of ground effect. Next on the list is terrain and after that is weather. If I’ve got a wide valley to circle and climb then I can deal with a slow rate of climb. If it’s a windy day or I expect downdrafts then that needs to be considered.

The CPA covers this stuff fairly well in the groundschool portion of their mountain flying course. Dunno what training is available in AZ, surely there are qualified instructors around.
 
AggieMike88 said:
Sorta surprised that hasn’t been converted into a digital product.
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They exist, I think, for some specific airplanes.

Setting up a performance table calculation is easy. Applying it to an airplane that is 60 years old, dirty, dented and flown by someone slightly less capable than a test pilot is what probably stops it from being more widespread.

Not every app that attempts to substitute electrons for brains works that well.
 
midlifeflyer said:
They exist, I think, for some specific airplanes.
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I think the "182P" app for the Skylane (created by Gyronimo, LLC) does what the slide rule does. And as you say, there are other versions for other specific aircraft.
 
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Clark1961 said:
The first rule of thumb to learn is that only 90% of the book max gross is used for high density altitude airport operations. Many folks have learned the hard way when they can’t climb out of ground effect. Next on the list is terrain and after that is weather. If I’ve got a wide valley to circle and climb then I can deal with a slow rate of climb. If it’s a windy day or I expect downdrafts then that needs to be considered.

The CPA covers this stuff fairly well in the groundschool portion of their mountain flying course. Dunno what training is available in AZ, surely there are qualified instructors around.
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I'm not sure I understand the meaning of that first sentance. Is this referring to gross weight in the POH being only 90% of actual GW, or that all values in the POH should be reduced by 10% in calculating, or something totally different? Sorry for being obtuse...just thought it sounded like info I ought to be aware of, but sure that I get what you are saying.
 
LongRoadBob said:
I'm not sure I understand the meaning of that first sentance. Is this referring to gross weight in the POH being only 90% of actual GW, or that all values in the POH should be reduced by 10% in calculating, or something totally different? Sorry for being obtuse...just thought it sounded like info I ought to be aware of, but sure that I get what you are saying.
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Do we agree that there is only one max gross weight for an aircraft?
 
Clark1961 said:
Do we agree that there is only one max gross weight for an aircraft?
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I sure believed it. But then again, I believed a lot that I have since found out wasn't exactly what I thought.

Are you trying to tell me it's deduct 10% off values, and if so, why? Is it because of the effects of age on performance in older planes?

I asked an earnest question, was hoping for a decent response.
 
LongRoadBob said:
I sure believed it.
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Good. Now take 90% of that value. <-- That is recommended as a limit for gross weight for our spam cans when operating at high density altitudes.

Your responses indicate confusion. One way to deal with confusion is to break things down to a simpler level. I believe we have accomplished that.
 
As for the "why" of it. Engine output for normally aspirated engines decreases at least 2%/1,000 ft of density altitude. There are also some loses in propeller efficiency. Limiting gross weight to 90% of max gross has seemed to work out as a practical way to improve performance.

Most of the take-off performance requirements for certification are for sea level operation. Some aircraft don't have performance tables for high density altitudes. The result is we end up with rules of thumb. Even though I have performance tables for the Dakota I still consider the rules of thumb for initial evaluation of a planned flight.
 
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Clark1961 said:
Good. Now take 90% of that value. <-- That is recommended as a limit for gross weight for our spam cans when operating at high density altitudes.

Your responses indicate confusion. One way to deal with confusion is to break things down to a simpler level. I believe we have accomplished that.
Click to expand...

Heh...ok. Fine.

I'm not confused, in general, I was confused by your sentence structure which wasn't quite clear. I'll take it on myself. If everyone else knew what you meant, fine. I worded my first response to that effect, apologies for not getting it, could you communicate with me what you meant, even if you have to dumb it down?

But the original, and even this response, is not clear writing, so yeah...confused. It's missing parts. It seems to be like coming into a conversation in the middle of it. It isn't clear. It is a haiku to me.

If everyone else got it, it was by a leap of understanding that I lack, being a student.
If it is important information, which it seemed to be, then it is equally important to be clear and that the person you respond to can understand it. "Now take that value" "qualifier"...then nothing.

I can onky guess. Zen koans are not a great way to learn about flight.
 
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Clark1961 said:
Looks like I can't help you. Maybe someone else can.
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Okay. Your call. I'm sorry for getting sarcastic, but it is a little frustrating for me. I was hoping just to get a clear understanding about your point. No hard feelings.
At least on my side.
 
Clark1961 said:
The first rule of thumb to learn is that only 90% of the book max gross is used for high density altitude airport operations. Many folks have learned the hard way when they can’t climb out of ground effect. Next on the list is terrain and after that is weather. If I’ve got a wide valley to circle and climb then I can deal with a slow rate of climb. If it’s a windy day or I expect downdrafts then that needs to be considered.

The CPA covers this stuff fairly well in the groundschool portion of their mountain flying course. Dunno what training is available in AZ, surely there are qualified instructors around.
Click to expand...
New Mexico Pilots Assoc also offers a high altitude/mountain course.
 
LongRoadBob said:
I'm not sure I understand the meaning of that first sentance. Is this referring to gross weight in the POH being only 90% of actual GW, or that all values in the POH should be reduced by 10% in calculating, or something totally different? Sorry for being obtuse...just thought it sounded like info I ought to be aware of, but sure that I get what you are saying.
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Colorado Pilots Assoc & New Mexico Pilots Assoc high altitude ground school recommend never loading an airplane more than 90% of gross weight. So for my 2500# cherokee, that means no more than 2160# max.
 
Let'sgoflying! said:
Many hi DA airports have at the fbo some very good advice, suggestions. Angel Fire, NM KAXX is one example; the lounge room is full of helpful suggestions. Same with long time pilots at these airport; lots of great advice goes unasked for.
Click to expand...

Maybe they offer the info because they get tired of picking up the pieces from folks who think it isn't important? Anyway to support your advice most folks are happy to discuss safe operations when ya call them or catch them during a quiet time.
 
murphey said:
Colorado Pilots Assoc & New Mexico Pilots Assoc high altitude ground school recommend never loading an airplane more than 90% of gross weight. So for my 2500# cherokee, that means no more than 2160# max.
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Creative math...not so sure I will buy into that answer but maybe it's some of that new maths stuff.
 
LongRoadBob said:
Heh...ok. Fine.

I'm not confused, in general, I was confused by your sentence structure which wasn't quite clear. I'll take it on myself. If everyone else knew what you meant, fine. I worded my first response to that effect, apologies for not getting it, could you communicate with me what you meant, even if you have to dumb it down?

But the original, and even this response, is not clear writing, so yeah...confused. It's missing parts. It seems to be like coming into a conversation in the middle of it. It isn't clear. It is a haiku to me.

If everyone else got it, it was by a leap of understanding that I lack, being a student.
If it is important information, which it seemed to be, then it is equally important to be clear and that the person you respond to can understand it. "Now take that value" "qualifier"...then nothing.

I can onky guess. Zen koans are not a great way to learn about flight.
Click to expand...
If the max gross weight of your airplane is 2900 lbs, use 2900-290 as the max gross weight for high density altitude ops.

The math is that simple. Considering some of the questions you have asked reflecting very complicated calculations based on Norweigian pilot certification requirements, I am not all that surprised by your confusion. Faced with a much of complex stuff, we tend to be triggered to expect everything to be complicated.

The 10% is one of those long-standing rules of thumb based on decades of practical experience in the mountain flying community.
 
Clark1961 said:
Creative math...not so sure I will buy into that answer but maybe it's some of that new maths stuff.
Click to expand...
2400
- 240 which is 10% of 2400
-----------
2160

unless, of course, you prefer a different number base, or wish to calculate using mass in slugs instead of the gravitational convention of pounds.
 
murphey said:
2400
- 240 which is 10% of 2400
-----------
2160

unless, of course, you prefer a different number base, or wish to calculate using mass in slugs instead of the gravitational convention of pounds.
Click to expand...
That is all fine but you listed 2500 as max gross.
 
Just saw the thread. No one has mentioned a critical item...

Learn how to lean your aircraft mixture for best power at altitude. Unless you’re operating a turbo-normalized aircraft, taking off full rich will rob you of considerable power.

Usually at a high airport on a hot day, you’re going to want to do a FULL power run up and lean for best power per your POH. And most older POHs will not mention this anywhere.

Calculate that you’re within power limits for best power first, most POHs will say 65% power or less, and on a hot day at a high airport you’ll be very close to 65% of normal sea level takeoff power.

If the book says you’ll be making greater than your power limit for best power leaning technique, do that full power run up, set best power lean, and then push forward on that mixture a small amount from that FULL power best power run up setting... and be ready to lean it back that tiny amount by pattern altitude, most likely.

Obviously, listen to your engine. Roughness will be a sign you’re way too rich or way too lean.

A vernier mixture helps immensely in this regard. One twist richer, one twist leaner. That sort of thing. Our airplane has one, and I love that feature for accurate mixture adjustments. But you can do this with sliding mixture controls too.

Side notes:

Many airplanes do NOT like being started full rich at high altitude on hot days... consult your POH, but ours doesn’t even like being started full rich in Denver. One inch back it catches and fires beautifully every time. On a hot start on many fuel injected engines you start at idle cut-off and advance to full rich as it fires. You may find you can advance until its running and stop there. Full rich may bog the engine down to the point of roughness. Every engine setup is different so ask around on your type before you go...

Additionally learn to lean aggressively for taxi. Fouling a plug will not help with getting best power in a full power run-up and may make you abort the entire flight until you can remove a plug and clean it. I have to come back another inch from best power lean on the ground to the point of a significant RPM rise when taxiing with our airplane.

There ya go... the book assumes best power for takeoff but often doesn’t elaborate well on what getting best power really means when the engine is starved for oxygen.
 
midlifeflyer said:
If the max gross weight of your airplane is 2900 lbs, use 2900-290 as the max gross weight for high density altitude ops.

The math is that simple. Considering some of the questions you have asked reflecting very complicated calculations based on Norweigian pilot certification requirements, I am not all that surprised by your confusion. Faced with a much of complex stuff, we tend to be triggered to expect everything to be complicated.

The 10% is one of those long-standing rules of thumb based on decades of practical experience in the mountain flying community.
Click to expand...

Thanks very much for that!
I totally agree on your assessment too. But that is very helpful!
 
denverpilot said:
Just saw the thread. No one has mentioned a critical item...

Learn how to lean your aircraft mixture for best power at altitude. Unless you’re operating a turbo-normalized aircraft, taking off full rich will rob you of considerable power.

Usually at a high airport on a hot day, you’re going to want to do a FULL power run up and lean for best power per your POH. And most older POHs will not mention this anywhere.

Calculate that you’re within power limits for best power first, most POHs will say 65% power or less, and on a hot day at a high airport you’ll be very close to 65% of normal sea level takeoff power.

If the book says you’ll be making greater than your power limit for best power leaning technique, do that full power run up, set best power lean, and then push forward on that mixture a small amount from that FULL power best power run up setting... and be ready to lean it back that tiny amount by pattern altitude, most likely.

Obviously, listen to your engine. Roughness will be a sign you’re way too rich or way too lean.

A vernier mixture helps immensely in this regard. One twist richer, one twist leaner. That sort of thing. Our airplane has one, and I love that feature for accurate mixture adjustments. But you can do this with sliding mixture controls too.

Side notes:

Many airplanes do NOT like being started full rich at high altitude on hot days... consult your POH, but ours doesn’t even like being started full rich in Denver. One inch back it catches and fires beautifully every time. On a hot start on many fuel injected engines you start at idle cut-off and advance to full rich as it fires. You may find you can advance until its running and stop there. Full rich may bog the engine down to the point of roughness. Every engine setup is different so ask around on your type before you go...

Additionally learn to lean aggressively for taxi. Fouling a plug will not help with getting best power in a full power run-up and may make you abort the entire flight until you can remove a plug and clean it. I have to come back another inch from best power lean on the ground to the point of a significant RPM rise when taxiing with our airplane.

There ya go... the book assumes best power for takeoff but often doesn’t elaborate well on what getting best power really means when the engine is starved for oxygen.
Click to expand...
1) Of course the issue of leaning was ignored because Clark & I were having a snotfest over my typo of 2500 vs 2400.
2) Golly gee, Unca Bob. Guy gets a CFI and becomes the #1 Encyclopedia of Aviation around here. ( which aint a bad thing )
( giggles ) [ hm....Apple emoticon keyboard does not work here....]
 
murphey said:
1) Of course the issue of leaning was ignored because Clark & I were having a snotfest over my typo of 2500 vs 2400.
2) Golly gee, Unca Bob. Guy gets a CFI and becomes the #1 Encyclopedia of Aviation around here. ( which aint a bad thing )
( giggles ) [ hm....Apple emoticon keyboard does not work here....]
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Ha! you may have ignored leaning because of snot. I ignored leaning because the OP mentioned it. The OP also mentioned max gross weight so the 90% rule got posted. Things went downhill from there. Far enough downhill and the 90% rule no longer is needed.

As for Zeld, adding 10% of 90% of MGOW or adding 10% of MGOW to 90% of MGOW? This is cornfussing.
 
At what density altitude does this 90% rule of thumb kick in? 5000'? 7000'? 9000'?

I think part of the reason this might be confusing is that 'high density altitude' means different things to different people. If you learn to fly in CO, 7000' DA is just normal; if you learn in New Hampshire, it's unheard of.
 
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