RVSM Certified

As I progress through my pre dual flight training study [by design], I have come across the concept of Density Altitude and its impact on aircraft performance. Right now, my focus is on the takeoff and landing segments of flight. I get the fact that density altitude is pressure altitude corrected for non-standard temperature, but that technical definition somehow leaves me still wanting more.

Apparently, density altitude can be impacted by temperature. An increase in temperature leads to an increase in density altitude which leads to a decrease in aircraft performance. Conversely, a decrease in temperature leads to a decrease in density altitude which leads to an increase in aircraft performance.

Additionally, I've read that density altitude can also be impacted by relative humidity with the same directional vectors as Temperature: raise this, lower that, lower this raise that. I also understand that when you combine the effects of both temperature and relative humidity, the impact on density altitude and by extension, aircraft performance, can be drastic.

But, here is what I don't understand: In high density altitude conditions, why does the delta between indicated airspeed and true airspeed begin to widen or increase?

Example: Normal departure airport field elevation is 3,000 ft msl. Atmospheric conditions are such that the HDA is 7,300 ft msl. So, the aircraft will now need more runway for the ground-roll. But, why does the aircraft also require a higher true airspeed/ground speed? I don't yet understand why.

I've read stories where pilots have flown into higher field elevation airports, spent some time on the ground, came back to the aircraft and loaded it up with full fuel, cargo and possibly a passenger or two, after a significant increase in the density altitude and then has very difficult trouble getting the nose off the ground during the ground-roll, having to either abort the landing, or ditching off the end of the runway.

This blows my mind and I'm trying to make sure that I don't do the same thing. Are these cases simple matters of just needing more airspeed? Or, are these matters a case where the HDA factor had grown well beyond the takeoff performance capability of these aircraft and no matter what the pilot did, the takeoff was doomed to end in an abort, regardless?

Thanks for the help in advance, guys.

RVSM Certified

The other question is this:

When I start doing cross-country flight planning, will I spend time investigating the historicity of destination and alternate airport ambient temperatures and relative humidity levels, so that I don't make the mistake of landing in an airport that I can't get out of? Or, am I placing too much emphasis on the effects of density altitude on real aircraft performance?

To me, it would seem like I would want to match the aircraft to the mission profile and not necessarily think that I can fly just anything into any situation. Or, am I being overly concerned?

BoredwLife

I will answer your last question. THIS IS A HUGE DEAL if you are flying in and out of mountainess terrain at an already high starting altitude. Summer time in the Rockies is a perfect example. I once flew a single out of Flagstaff airport in the summer time with a DA of about 10,900 feet. If you want an example of performace take you A/C up to 11,000 feet and slow to your rotation speed and add full power and try to climb. That is the exact same performace you will see trying to climb out just slighly better.

USMCFLYR

RVSM -

I worry about HIGH, HOT, AND HUMID in my airplane - so I would certainly advise you to worry about any aircrft that you might end up flying on a X/C

I remember well 29 Palms Expeditionary Airfield in August with a temperature hovering somewhere around 115 degs, with full fuel and a heavy payload and watching the end of that 8,000 foot, AM-2 matting runway screaming toward me and wondering if this thing (my airplane) was actually going to fly or not!

USMCFLYR

rickair7777

High altitude airports in the summer routinely eat GA aircraft, this real stuff and not trivial.

rickair7777

Indicated airspeed is generated by air molecules impacting the pitot tube. The faster the true airspeed, the more molecules impact the pitot.

However as altitude increases, the air becomes less dense so there are fewer molecules impacting the pitot, so indicated airspeed drops even though true airspeed remains the same.

A high density altitude does the same thing as a high MSL altitude...less dense air.

Operationally, you normally land and takeoff at the same IAS regardless of density altitude. The IAS measures molecules, and molecules make the wing fly so a given number of molecules impacting the pitot corresponds to a certain number flowing over the wing.

At high density altitude, a higher TAS is required to achieve the needed IAS. This results i higher groundspeed, which means a longer runway is required (for both TO and LDG).

As for humidity...air is a mixture of molecules, at a given pressure (altitude) a certain volume of air contains a fixed number of molecules. These molecules can be any of several gasses, including water vapor. Air is mostly made up of oxygen and nitrogen molecules (O2 and N2)...these molecules are actually heavier than a water molecule because H2O has two hydrogen atoms which are the lightest atom on the periodic table. Since the presence of water molecules means less N2 and O2 molecules, that volume of air is going to be less dense. This jeans it will have less effect on the pitot tube and the wing.

Density altitude also effects propulsion...less O2 for the engine to burn, and the prop is just a spinning wing, so it suffers too.

forumname

See what you're saying, but I'm not sure if that's the best way to illustrate the point.

Even at a high DA airport, on the take off you STILL have energy and momentum going forward.

In your example, you are slowing the aircraft, decreasing energy, then trying to reverse that momentum and energy. There is the additional energy being consumed trying to reverse the trend, further decreasing the climb performance.

SunDog

for the take off at a high density altitude, the air is thinner so you have less mass being moved around by the wing and therefore less lift at a given (true) airspeed. likewise your propeller is producing less "lift in the forward direction" which means less thrust.

Also, your engine is getting less oxygen and so your engine produces less power to turn the prop.

Indicated airspeed is measured by the difference between dynamic pressure and static pressure. the density is on both sides of that equation and so it cancels out. therefore, you get the same amount of lift for a particular indicated airspeed regardless of density, but it takes a lot more speed to make that lift at a high density altitude.

humidity affects density, but it is a pretty small factor. the thing to worry about is heat.

you don't even need to be full fuel and heavy to have problems due to density altitude. once in an older 172 with only two people on board and half tanks I had to abort takeoff on the big runway at PRC, 5045 feet and 90 degrees.

RVSM Certified

Aaaah, yes. Thanks, I can see it now. Got it!

Now, what about the IAS -vs- True delta that comes with an increase in DA? And, how much flight planning time do you spend going over the details of the atmospherics [potential HDA problems] of your destination and alternate airports? Is it something you consciously handle at the flight planning phase, or do you just deal with it, if it becomes a problem once you get to your destination airport?

USMCFLYR

Performance charts for your particular aircraft will tell you everything you need to know.

USMCFLYR