How Fast Is Fast Enough?

Taking off in a T-38 Talon usually requires two engines, but in the case that one of those engines fails, pilots and engineers here are working to determine the safest single engine takeoff speed for this pilot training aircraft.

Edwards' test pilots last completed single engine takeoff speed, or SETOS, testing on the T-38 in 1966; recent modifications to the aircraft's propulsion system increased takeoff performance, which now requires additional SETOS data.

The SETOS testing ending today, culminates more than a year of testing that started in October of 2004.

Single-engine takeoff testing is considered high-risk because test pilots are shutting down an engine during takeoff, said 1st Lt. Chris Previte, 416th Flight Test Squadron flight test engineer for the SETOS program.

While both engines are running during the initial phase of the takeoff, the test is designed to simulate the loss of an engine after the initial takeoff roll, said Roger Tanner, 416th FLTS project pilot for the T-38 single engine takeoff test.

"We accelerate on two engines out to our critical engine failure speed at approximately 130 knots, and then we pull the right engine back rather quickly and shut it off," Tanner said. "We check the health of the left engine and then go ahead and continue to take off."

This is to simulate an engine failure, he said.

"It's not a place where aircrews want to be when flying this aircraft," Lieutenant Previte said. "You have about one-half of the thrust, and there is the possibility that you may not be able to climb out, which is why we're doing the testing to begin with."

It's a matter of safety, Tanner said.

"We want to do the test here in a controlled environment before someone ends up having to do it in a real environment with a real engine failure," he said.

To mitigate the inherent risks of single engine takeoffs, Lieutenant Previte said a lot of planning went into the test protocols. Testers also have a second pilot in the back seat of the T-38 to monitor the remaining engine's health and to assist the primary pilot if the second engine were to fail.

"We've taken all the precautions we possibly can to lower that risk," he said. "For example, we have a 15,000-foot runway, plus the lakebed adds another 24,000 feet. In case Tanner can't take off for some reason, he can set the aircraft down on the lakebed."

As Tanner explained, "There are not as many redundant systems available in case there is an engine failure, but we've mitigated most of those risks by having a second pilot in the back seat to watch the health of the good engine and back me up on airspeeds and procedures. As soon as we get the gear and the flaps up, we start the right engine, which is the engine that we're initially failing."

One of the main purposes of the propulsion modernization program is to allow the T-38 to have increased takeoff performance when Category III air temperature conditions exist. For example, a T-38 at Randolph Air Force Base, Texas, is restricted from taking off when the outside air temperature exceeds 95 degrees Fahrenheit. Above this temperature the existing runway length there does not allow for the aircraft to takeoff or stop safely if an engine were to fail.

The testing being accomplished here is designed to prove that the modifications to the T-38 have improved the aircraft's performance," said 1st Lt. Dan Urban, 416th FLTS project engineer on T-38 SETOS program.

"Our charter was to get air temperatures as high as we could," Lieutenant Urban said. "What we've done is test to temperatures as high as 110 degrees Fahrenheit, which is... maybe 20 or so degrees hotter than what we've done in the past."

There is a certain temperature at which the aircraft will no longer be able to produce enough thrust to safely get the aircraft airborne with the loss of one engine, said Maj. John Kanuch, 416th FLTS test pilot, who is a safety observer pilot for T-38 SETOS testing.

"Because most of our pilot training bases are located in the southern United States, it will give them more days in the year that they can use their aircraft," Major Kanuch said.

"The Air Force puts some improvements on the [aircraft], and the engineers try their best to predict what those improvements might allow for, but the bottom line is we don't know until we actually fly the aircraft at those conditions," he said.

Since the 2001 T-38 modifications began, engineers here have been compiling data to accurately gauge what the minimum speed is to takeoff safely if an engine were to fail.

"The model in the past has always extended out beyond the range where there was data to support it," Lieutenant Urban said. "What we've done is gone out and collected data in that extended range to prove that these aircraft can take off at these hotter temperatures."

The SETOS testing is really going back to validate what engineers predicted the single engine takeoff performance to be, Tanner said.

"This is a kind of graduation exercise to prove the performance of the testing we've been doing for the last four years," Tanner said.

The goal of the entire program, Major Kanuch said, "is to make sure the men and women out in the field have the most up to date and current information we can provide them on this T-38 trainer aircraft.

"It's our job here at Edwards to try to figure out and validate the data to make sure the information operational pilots are using is the best they can possibly have," he said.

(ANN salutes Article by Tech. Sgt. Eric M. Grill, Edwards AFB Public Affairs)

FMI: www.edwards.af.mil