Density altitude and the third 'H'
FAA publication 61-23C, Pilot's Handbook of Aeronautical Knowledge, states that "Air density is perhaps the single most important factor affecting airplane performance." The effects of air temperature and altitude on air density are discussed in some detail, and methods are given for calculating density altitude using these variables. The impact of humidity on air density, however, is hardly considered. Humidity is dismissed with the statements "any given volume of moist air weighs less--is less dense--than an equal volume of dry air" and "keep in mind that high humidity will decrease airplane performance."
I am curious as to why more emphasis is not placed on including the impact of humidity on density altitude calculations. Handheld aviation computers that I have seen, from the old manual E6B to current Jeppesen electronic units, do not include humidity in density altitude calculations.
Thanks,
Mr. B.
Greetings Mr. B.:
An increase in humidity produces an increase in density altitude, but it's not often dramatic. Increasing the relative humidity of standard-temperature air from 50 percent to 100 percent usually doesn't increase the density altitude by more than 100 to 150 feet. Now, if the air is significantly above standard temperature, then the increase in density altitude can be as high as 600 feet.
For example, if the air temperature at 5,000 feet is 80 degrees Fahrenheit with a 0-percent relative humidity, then the density altitude is approximately 7,445 feet. Increase the humidity in the example above to 100 percent and the density altitude jumps to 7,957 feet. That's an increase of 514 feet caused by a 100-percent increase in relative humidity at these higher temperatures.
It's also a pretty unlikely scenario. Eighty degrees F is substantially above standard at 5,000 feet. If we assume standard temperature at 5,000 feet (about 41 degrees F), then a similar increase in relative humidity produces an increase in density altitude of only 129 feet. The reason for this is that cooler air holds less water than warmer air. Therefore a 100-percent increase in humidity in cooler air produces a less significant change in density altitude. When the air temp is significantly above standard to begin with, then larger changes can be expected. This may be one reason that humidity isn't considered a significant factor in airplane performance.
Perhaps another way of thinking about this is that an increase in humidity only matters when it matters (and I don't mean that in a sarcastic way, either). I suspect when a person bends an airplane because of a density altitude problem, the difference humidity makes in air density probably wouldn't have made any difference in the outcome of the event. As I see it, anyone cautious enough to do a density altitude computation before takeoff probably is already conservative enough to modify his behavior to ensure a successful takeoff (perhaps by offloading fuel; taking one passenger at a time to a lower density altitude airport, then returning for the lonely guy; etc.).
Then again, with the computing power of modern handheld flight computers, there's absolutely no reason why humidity can't (and shouldn't) be considered as a factor in computing density altitude. Yes, the equations can be a bit complex, but that's what transistors are for.
If you'd like to see for yourself how humidity affects density altitude, take a look at Richard Shelquist's density altitude calculator page.
Sideways or forward?
Dear Rod:
Is it a side slip or a forward slip when a pilot transitions from a crab to cross-controlling on landing to counter a crosswind? At what altitude above the runway is the kickout accomplished?
Thanks,
Harry
Greetings Harry:
It's a side slip when you transition from a crab to a cross-controlled condition for crosswind landings. This is, however, simply a matter of definition since the airplane doesn't know whether it's in a side or forward slip.
Regarding the kickout, let it be known to the audience that you're referring to using a crab condition to compensate for the crosswind and applying rudder to straighten or "kick out" the airplane just prior to landing. This isn't a maneuver used by an instructor to handle students who ask difficult questions, either. The secret here is to align the airplane's longitudinal axis with the runway centerline (i.e., kick it out) just before the wheels make contact with the runway. The closer to the runway you align the airplane, the less chance there is that you'll encounter additional crosswind drift before touchdown. So the goal is to do this as close to touchdown as possible. On the other hand, leave enough margin so that you don't land with sideways stress on the gear.
Pulling the mixture not recommended
Dear Rod:
Is it safe to simulate an engine-out by chopping the mixture to full lean? And will the prop stop if my instructor simulates engine failure this way?
Thanks,
R.B.
Greetings R.B.:
Simulating engine failure in a single-engine airplane by chopping the mixture to its idle-cutoff position isn't a recommended practice in any aviation literature of which I'm aware, be it a textbook, POH, or written literature such as an advisory circular. Nor, I suspect, will you find the practice recommended at any FAA safety seminar, by any FAA inspector, or any other recognized aviation expert. Now, this doesn't mean that you'll perish if your instructor pulls the mixture to idle cutoff when aloft. It does mean that the chances of having an engine stop and not be able to to get it started again (or at least started in time) are increased.
Realistically speaking, you often have to slow the airplane way down, sometimes near stall speed, to stop propeller movement in many small airplane engines. In many smaller airplanes, your typical best-glide speed often keeps the propeller windmilling for quite awhile, as long as internal engine damage isn't present. This is why it's better to pull the throttle to flight idle for a more realistic engine failure simulation.
Then again, if your instructor wanted to simulate a stopped propeller, he or she could apply just enough power to create a zero thrust condition. This simulates the reduced drag of a stopped propeller, something that's more likely to happen if mechanical damage were to cause the engine failure.
Rod Machado is a flight instructor, author, educator, and speaker. A pilot since 1970 and a CFI since 1974, he has flown more than 8,000 hours and owns a Beech A36 Bonanza. Visit his Web site.
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