Think of it in terms of instrument calibration, the the A/S indicators is a mechanical instrument that operates on pressure differential calibrated at one altitude.

Now, the higher the density altitude, the fewer air molecules there are to push against the indicator's pDiff membrane.

If the instrument could sample the actual number of ambient molecules at any altitude/temp and calibrate the airspeed indicator on the fly, you wouldn't have to worry about it.

Fortunately engineers have done the math and that's what performance charts do for you in the absence of a self calibrating airspeed indicator.

Even with proven performance charts and sound operating practice, I've seen the far end of high altitude runways at much lower altitude than I'd like. This is a lesson you really want to understand, to ignore it can ruin your day.

It might help a bit to think of IAS as =not= being a direct measure of speed. All it's measuring is air impact on the pitot tube. As others explained, you need more impact of less dense air in order to get the same needle movement on the ASI than what you would get at lower D-Alt. IOW, more true to get the same indicated.

It's like everything else in flight planning - you do less navigational planning over a familiar route that you do over an unfamiliar one.

A pilot unfamiliar with flight in high density altitude conditions will probably give more thought to a high-altitude takeoff or landing than one who does it every day, just as a land-locked pilot will take a little more time planning for that over-water route than one familiar with flying over water.

But those differences are mostly operational. D-Alt is really just another condition of flight and is accounted for in normal flight planning. Takeoff and landing distance charts take D-Alt into effect. Enroute performance charts do as well. It's not really a "problem to deal with."

Nice discussion.

RSVM asked if it is possible to blunder into an airport where you simply can't get back out due to the high field elevation and prevailing density altitude. This is very much true, and not only for piston aircraft but for jets with all their additional power. Any business jet pilot who regularly flies into Aspen can confirm there are times of the day, weather situations, Departure Procedures, and loadings of the aircraft that are impossible to work with at times. I took a Citation Ten class a week or two ago and we examined some situations where this most powerful of business aircraft is completely grounded for days, even under minimal loadings, at certain high altitude airports like Aspen. There are a lot of factors. And yet another wrench in the works to consider is that many of the high altitude airports do not have multiple runways to choose from. This means you will suffer additional performance losses due to tailwinds and crosswinds at those locations.

As the marine said, the performance charts in your POH take it into account.

a good practice is to run all of the performance numbers after you get your weather briefing, based on both the current temperature and the expected temperature. Even for local flights, I will run the numbers at my home airport if the temperature is outside of a range i have flown recently. (or if it is more than just me in the plane)

I usually do the takeoff and landing numbers both with 0-wind and max tailwind conditions to get the worst case numbers. however, as a student, it is good practice to work those performance problems as many times and with as many different variables as possible because you learn to do them quickly but also because you get a good feel for how different conditions affect flight characteristics.

True. And, like Aspen, there are a good number of mountain airports that are one-way. You land in one direction and take of in the opposite direction, mostly because of rising terrain.

And because mountain airports are build where they can fit, they may not be aligned with the prevailing winds. A great example of this is Angel Fire, NM (KAXX). The runway is North-South because that's where the terrain gives you enough room - the same terrain that results in the wind almost always being a crosswind. For example, pulling up the current METAR.
That's pretty usual for this airport. The runway is 17-35.

But those are mountain issues that can exist at even some much lower mountains.

Density altitude and it's performance-sapping effects take place even in flat terrain and even at low true altitudes. My first exposure to D-Alt was in Connecticut! Only 253 msl, 7B9's 1800' runway meant that we considered density altitude in a 152 on those 90°/90% humidity summer days.

Business jets have the advantage of being equipped with performance calculating software. C750s have two Honeywell FMS units in the cockpit to assist with performance calculations. In addition, the Ten has another software called CPCalc which tells you what the airplane can do under prevailing conditions at the time. You can use it before leaving your hotel room.

Statistically general aviation is the most susceptible to risks associated with high altitude operations, because they have less resources to draw on. No excuses, but experience shows that GA is the more vulnerable segment. And as Noy says, they do not need to go to Aspen to experience problems with hot and high performance.

This is a great thread....

But some folks learn better by hearing than by reading....I would point you to a great podcast

www.thefinerpoints.net

This CFI does a great job discussing private pilot matters, passing the checkride, all the way too aerobatic flight. The music isn't exactly my style, but the matters discussed are great.

......for a new airplane, that hasn't been bent or tweaked, with a with a new engine developing max horsepower/thrust for the ambient conditions, with a test pilot at the controls.

So you add another 10% if your're worried about it. Are you advocating NOT using performance charts?

USMCFLYR

Easy there tiger, can you show me where in my post I WAS advocating that? What I was getting at is the charts are done in a "best case scenario" situation.

If you want to read more into that, it's YOUR prerogative, but you're WAY off.

Also, who's to say that 10% is ALWAYS the best fudge factor? I did all of my training/instructing out of high DA airports, and a lot of flying into places like ASE, AVO, LXE, GUC, HDN, etc. In the summer, and places like that with training airplanes that have a high utilization rate, 10% really wouldn't be enough.