April 8, 2013
By Dan Namowitz
Quick—what’s the textbook definition of true airspeed? Do you keep track of TAS in flight? Can you discuss a circumstance in which TAS would trigger a mandatory-reporting requirement during an IFR flight?
TAS, says the book, is calibrated airspeed “corrected for nonstandard pressure and temperature.” The two are identical in standard atmosphere at sea level. Under other conditions, find TAS by correcting calibrated airspeed for pressure altitude and temperature.
TAS is what your performance charts provide when you look to see what you’ll get out of your engine at a given altitude and power.
It is, yes, your true (not indicated) airspeed.
Pilots sometimes turn true into a verb. “A typical training aircraft at 110 KCAS in a standard atmosphere at 8,000 feet trues at 124 KTAS,” said this Flight Training magazine article about two systems for calculating true airspeed.
If this is tickling a memory from instrument training, look up mandatory reporting requirements for IFR ops. You’ll find one requiring that, at all times, you must report a “change in average true airspeed (at cruising altitude) when it varies by 5 percent or 10 knots (whichever is greater) from that filed in the flight plan.”
Like many pilots, you may have fallen into the habit of filing the same true airspeed for your aircraft on every flight, regardless of numbers produced by flight planning. Fortunately, many aircraft have an airspeed indicator with a TAS ring, allowing the pilot to check TAS at a glance. And a mechanical solution is always possible.
If a 5 percent change is reportable, consider the reaction a 50-percent deviation provoked in a futuristic exception that demonstrates the rule.
“The flight plan usually shows a TAS of 130, however observed ground speeds correspond to approximately 190 KTAS while enroute,” said an ATC staffer in a report to the Aviation Safety Reporting System, adding, “If these flights are to be handled as ‘normal’ operations, then off-airways routing in areas without RADAR or RCAG coverage are not acceptable where it harms our ability to move IFR traffic in and out of airports. TAS errors on the order of 50 percent are not acceptable.”
Turns out the offender was an unmanned aerial vehicle—or anyway, its operator. The ATC staffer added this appeal for restoring order through better planning: “These operations have been allowed to deteriorate, with half-baked verbal briefings becoming the norm.”
Dan Namowitz is an aviation writer and flight instructor.
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