August 2004
Well here comes summer once again. If you are based east of the Mississippi, that means high temperatures and low flight visibilities, thunderstorms and high density altitudes. Here in sunny South Florida flight vis doesn’t get too bad, but temperatures on the runway can exceed 140 degrees F.
The effects on aircraft performance in high temperature situations are the result of density altitude. Some aircraft are actually prohibited from operating in these high temperatures. Some of those limitations are based on the fact that there is no performance data for pilots to plan from. The Citation I recently typed in is prohibited from takeoff in temperatures above 39 degrees C ISA.
By definition, density altitude is the measure of air density. Do not confuse it with pressure altitude, true altitude or absolute altitude. It is not to be used as a height reference, but as a determining criteria for the performance of an aircraft.
In simple terms, it is the altitude the aircraft “thinks” it is at. Molecules of warm air are further apart than in cold air.
Furthermore, in high humidity situations, since two molecules can’t occupy the same space, water molecules displace air molecules.
So why would I care here in South Florida where the highest surface elevation is the on ramp to the freeway? Even on a day where the barometric pressure is 29.92 in of mercury, when the surface temperature reaches 140 degrees F, which it can easily do in the heat of the day, the aircraft “thinks” it’s at 5,000 feet.
Look at the performance section of your aircraft flight manual and see what it says about temperatures above standard and how they affect aircraft performance. For time fuel and distance to climb, typically you need to add 10 percent for each 10 degrees C above standard.
If a standard day is 15 degrees C, and 140 degrees F is about 60 degrees C, you need to be adding about 45 percent to your time, fuel and distance to climb figures. This becomes especially important when flying a departure procedure with non-standard departure minimums.
A perfect example of what I am talking about is the TATES TWO and the WILTT TWO departure from Altoona, Penn. The field elevation is about 1,504 feet. It is about 11.5 miles from the field to Tates intersection, which you must cross at 5,000 feet. That means you must climb 3,500 feet in 11.5 miles or 318 feet per nautical mile.
To achieve that in a typical high performance single you would need to see a climb rate in excess of 525 fpm in a 90 knot climb. Can your aircraft do that when it “thinks” it is at 6,800 feet already?
Remember that the performance figures in your book are for a factory new aircraft, flown by a factory test pilot in what can only be described as “factory air.” Real world experience says add 10 to 15 percent simply because your engine and airframe are not factory new and you are not a factory test pilot.
So you don’t need to be flying in Colorado to consider the effects of density altitude on your flight planning.
Not all obstructions are manufactured by Mother Nature, either. Here in Miami, the Miami 6 departure features nonstandard departure minimums so you clear the condominiums on the beach, just east of the airport. They put the obstruction height at 1,300 feet (not because the building is so tall, but for noise) then you add 50 feet to that. You need to cross at 1,350 feet. Can you do that on a really hot day in your aircraft?
Density altitude has a minimized effect on turbocharged aircraft because unlike their normally aspirated cousins, these engines continue to make full rated power up to their critical altitude, which is typically in the teens. But the wing still “thinks” it is operating at the high-density altitude.
One trick jet operators use when it is hot is to reduce the weight of the aircraft. This is most easily accomplished by limiting your fuel load. Just because you can go with full tanks doesn’t mean you have to.
Another trick is simply to go early or late in the day when it just is not so hot. (I am personally convinced that is the primary reason they invented daylight savings time!)
It is equally important that you realize that your engine thinks it is at a higher altitude as well and you must lean the engine for best power (typically about 125 percent rich of peak EGT) or max RPM (if you are flying a fixed pitch aircraft) before you start the takeoff roll.
Remember that it is not just take off performance that is affected. Your landing distances increase as well. Even though you are flying the same approach speeds as indicated by the airspeed indicator, you true airspeed is significantly higher as is your ground speed, which translates into longer landing distances.
Also consider the ramifications of high-density altitude on your aircrafts service ceiling as well. I typically run my normally aspirated singles at 10,000 feet in the summer. I have seen density altitudes at that altitude in excess of 15,000 feet. That is higher than the service ceiling on many light singles.
In short, density altitude is “performance altitude.”
Another consideration is density altitude’s relationship to your body. Not only does the aircraft “think” it is at a higher than indicated altitude, so does your body. While it is perfectly legal to fly all day at 10,000 feet indicated without the use of supplemental oxygen, you simply can’t fool Mother Nature. That headache you experienced is not a sign that you need a new headset; it is a sign that you are hypoxic.
Portable O2 systems are so easy to use, so affordable and small in size that there is just no excuse for not owning and using one. I carry a little 9 cubic foot system that is so compact it fits in my flight case. With the use of nasal cannulas, it will provide two pilots O2 for nearly eight hours at the altitudes I fly. That is roughly double the amount of fuel I have on board.
The FAA has been watching density altitude-related accidents for a long time. A few years back, at some of the airports that hosted more than their fair share of these events, the feds installed electronic density altitude signs at the end of the runway, advising the current density altitude to any pilot who cared to read it. I thought it was a great idea; they thought it was too expensive.
Nowadays, most automated weather now gives density altitude reports, and virtually every GPS has a density altitude computer built right into it, so there is no excuse for not knowing what the density altitude is.
When considering the effects on your aircraft’s performance, error on the conservative side. Take the extra two minutes to break out the book and figure out the takeoff and landing distances as well as the expected climb performance. While you are at it, figure out your cruise performance as well.
Optimum altitudes for normally aspirated aircraft are around 7,000 feet. If the aircraft thinks it at 14,000 feet you will find that it will be hard pressed to make even 55 percent of rated power.
About the only truly great thing about high-density altitudes is that come next winter, you will simply marvel at the climb rate and cruise speeds your aircraft is capable of.
Michael Leighton is a 3.000 + CFIIMEI/ATP as well as an A&P mechanic and former FAA Accident Prevention Counselor. He operates an air charter company in South Florida. You can reach him via e-mail at av8tor0414@aol.com.
