Page 34 - May22T

On Final
by David Miller
NTaking Your Temperature
o, this is not a piece about the importance of is also really hot, your performance will decrease even flying healthily. Instead, it’s a reminder of the ef- more. The same goes for your airplane. Remember, I fects of temperature on the performance of your passed a college course in this stuff.
airplane – especially turbine-powered jets. And specifi- cally, the temperature at cruising altitude.
I am an expert on the subject because I passed a three- hour credit course on “climatology” in college.
Of course, any airplane’s performance is affected by temperature, density altitude, weight, etc. Years ago, the aviation community decided to base performance data on a “standard atmosphere.” I guess they all got together for drinks and agreed on exactly what that would look like, but it allowed each airplane’s performance to be mea- sured equally. They called it the International Standard Atmosphere, or ISA. Each altitude was given a standard temperature. If on a given day the actual temperature is warmer than the standard, we call it ISA plus “X.”
And here’s where that temperature thing becomes important.
If you are running a foot race in Aspen, Colorado, the high altitude lessens your performance. If the temperature
Some airplanes are affected more than others.
In my C90 King Air, a really warm day, say ISA + 10 degrees C (Celsius), had no significant effect on fuel burn or performance mainly because the King Air flies at lower altitudes, say 25,000 feet. But your average single-pilot jet cruising at 41,000 to 45,000 feet is a different animal. Jets must cruise at high altitudes to achieve the range and fuel burn advertised in their brochures.
To compound the issue, some jets have more “excess” thrust than others making it easier for them to achieve performance numbers even in ISA+ temperatures.
For example, the Citation CJ3+ is “overpowered” in that its engines can meet performance numbers even in ISA+ situations. My sweet little Mustang, not so much. The engines on the Mustang perform well at ISA + 0 temperatures but warm up the air just a little bit, and the airplane is not happy.
A recent flight home from Columbus, Ohio (KCMH) illustrated this dramatically. My flight up from Dallas with CJP CEO Trent Corcia was a nonstop breeze in slightly over two hours, with a 70-knot tailwind. The temperatures along the route at FL 390 were ISA + 0, a standard atmosphere. The Mustang was very happy. Less than 24 hours later, an approaching cold front totally changed the picture. My planned nonstop return was possible only if I could cruise at FL400. Temperatures in the climb out of Columbus were forecast to be ISA minus 5. The Mustang loves that situation. But climbing through FL240 the temps were forecast to increase to ISA plus 9!
I knew from about 1,000 hours in the airplane that it would not be possible to climb up to FL400 in such warm air. And the increased fuel burn at a reachable altitude of FL360 would make a nonstop trip impossible. I stopped in Little Rock for fuel.
Take your temperature before every flight. Fly safe.
David Miller has owned and flown a variety of aircraft from light twins to midsize jets for more than 50 years. With 6,000 plus hours in his logbook, David is the Direc- tor of Programs and Safety Education for the Citation Jet Pilot’s Safety Foundation. You can contact David at davidmiller1@sbcglobal.net.
32 • TWIN & TURBINE / May 2022