Density Altitude Flight Planning
 

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. :)

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?

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.

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! :eek:

USMCFLYR

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

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.

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.

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.

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?

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

USMCFLYR

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.

Forumname -

No reason to get defensive and call someone "tiger". I asked a question - that is why there is a '?' at the end of that sentence. I asked if you were advocating that. I'm sorry you took that as some sort of challenge.

Back on track. If you are talking about certain situation or airports with special requirements then maybe 10% is not enough. Of course then you are saying that your performance charts (which already take into those factors into account) are more than 10% inaccurate. I generally find my performance charts to be optimistic so I understand what you are saying about the reliability of the information. In any case - MY experience tells me that adding about 10% to numbers will work pretty well in MY aircraft. Your mileage may vary.

USMCFLYR

Not being defensive. But your post had an accusational tone that I DID advocate it, like I said, not the case.

That's what I was getting at in the FIRST place.

When I started with my airline, we used to fly Metroliners in to South Lake Tahoe airport. Not only did we have to concern ourselves with takeoff and landing performance but also with things like max balked landing weight, etc,. Tahoe presented an interesting additional problem: One could takeoff to the south - in to the wind, but also toward the mountains which would provide much entertainment event of an engine failure, OR takeoff north toward the lake, but with a tailwind. Multi-engined airliner crews have to show (on paper) that every flight can be conducted with an engine failure at the most critical time, and still meet performance guidelines.
In the summer, we'd fly 19 passengers TO Tahoe - but only 3 FROM Tahoe...

So, does this mean that all those pilots out there that I'm reading about having problems launching from HDA airports, ditching out at the end of the runway, taking-off and then plowing into something direct in front of slight elevated above the end of the runway, or just taking-off and stalling because they pulled back too far and too soon on the yoke; did not read their performance charts?

If that's the case, then I'm good to go because I'll always read mine!

However, can there really be that many pilots out there who simply "forgot" about the importance of double checking their performance charts before beginning operations at an airport in HDA? If so, is that a training issue that I can get my instructor to focus on a bit more?

I'm mean, based on what I'm reading, something is broken here. Either the student did not get the training at all, or the student got the training and somehow felt that it did not apply to him/her, or somehow the student simply forgot how important it was because every summer I read about the same kinds of accidents.

Heck - they are almost predictable at this point each summer! I just want to know if there might be something in my training that I can tweak or look out for, so this ghost of a problem does not come back to haunt me, either later in my advanced training, or later in my flying career once I've moved on to bigger, higher performing aircraft.

As usual, thanks for the input.

You said: more of less in order to get the same. Bingo, that did it! That's what my brain needed, to see a relationship between "less" [molecules] and "more" [impact] resulting in "same" [airspeed]. Nicely done! :)

Now, I can begin toss this thing around in my head and play with it a little bit. In fact, now that I understand what you guys are talking about here, I think I can restate it a different way:

What you guys are really saying is that the faster ground speed reduces the time interval ("t") between air molecule strikes inside the pitot under HDA conditions. Thus, the reduction in "t" (time) between molecule strikes inside the Pitot, actually fools the ASI into "thinking" that the aircraft is going faster when in fact its actual ground speed could be getting slower [under certain circumstances]. The inverse would also be true. Increase "t" between strikes and the ASI is fooled into thinking the aircraft is getting slower when in fact its actual ground speed could be getting faster [again, under certain circumstances].

Hmmmm. Tossing this thing around in my head even further, presents two additional questions that seem very troubling to say the least:

Question 1: Scenario A

What happens, if during the last quarter of the ground roll during a take-off in HDA conditions, there is a fairly strong gusting headwind that progressively increases non-linearly in relation to the progressively increasing ground speed, forcing the pitot to register a higher IAS on the ASI and the pilot begins to pull back on the yoke or stick too soon, just as the straight-on headwind gust abruptly comes to an end? :eek:

Now, in this scenario, the aircraft's ASI showed the attainment of the correct rotation airspeed somewhere after 3/4's of the way into the normal ground-roll [let's say around 82% of the expected ground-roll]. The pilot responds to the ASI by pulling back, then the gusting headwind stops, the ASI drops back to "normal" for this segment of the ground-roll at this HDA, leaving the pilot hanging high and dry with a raised elevator now producing more induced drag slowing the aircraft down even more!

Now, on a "standard day" this would seem to be less serious. But, on a significant HDA day, this seems like it would be enough to really cause some problems for the pilot. Yes/No?


Question 2: Scenario B

A progressive, non-linear gust build-up on the nose of the aircraft during approach to landing [a very stealthy build-up, not a single blast] under HDA conditions. Time interval between strikes in the pitot is reduced, fooling the ASI. Pilot sees this mirage [but does not recognize it as such] on the ASI and either reduces power or pitches up to bleed off some airspeed [now you understand why I asked those Pitch/Power/Trim questions!]. Then all of a sudden, no more headwind gust, ASI drops to "normal" for this HDA condition, leaving pilot with either too low a throttle setting or too high a pitch attitude - or both.

What does the pilot do?

Again, a "standard day" is one thing, but doesn't the stall speed increase as the DA increases due to the need for higher TAS to maintain the same IAS? And, if that is true, is not this scenario a rather dangerous one to have to deal with?

Rememer, the key here in these two scenarios is the progressively [stealthy] increase in the rate at which the headwind is gusting relative to the aircraft's opposing direction ground speed, followed by a rather fast/quick fall off of the gust component after the pilot has already responded to the uptick on the ASI.

As usual, thanks guys for the feedback.

Yes to all of your questions. Some were not taught properly, some were taught and forgot and others were taught and just didn't think that the law of physics applied to them. If you are a careful planner and do everything that you are suppose to, every time, then you will be find.
Headwork and decision making can not always be taught. You have to have the WILL to apply it.

USMCFLYR