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NTSB recommends new round of Tomahawk tests

By MICHAEL SWEENEY

VERO BEACH, Florida - The controversial and unpredictable stall/spin characteristics of the Piper Tomahawk are finally going to be fully tested by the FAA - 20 years after the agency certified the airplane.

The National Transportation Safety Board (NTSB) called for the tests after several Tomahawk crashes that were caused by unintentional spins, and after finding no evidence that Piper ever completed the full series of stall tests that are required as part of the FAA certification process.

The NTSB also noted that the limited stall/spin testing of the Tomahawk that Piper reported performing in 1977 was done in a pre-production aircraft, which may have exhibited significantly different stall characteristics than exist on production airplanes.

Pending completion of the new round of certification tests, the NTSB asked the FAA to immediately require that any slow flight and stall training in the PA-38-112 Tomahawk be conducted at or above the minimum altitude currently specified in the pilots operating handbook (POH).

The Tomahawk POH recommends that spins be started at altitudes high enough to recover fully at least 4,000 feet AGL, to provide an adequate margin of safety.

The NTSB also asked the FAA to inform pilots of alternative methods of recovery from an inadvertent, possibly flat, spin.

One of those "alternative" recovery methods calls for the occupants of a Tomahawk to shift their weight fully forward against the instrument panel to bring the nose down. This highly unorthodox method of spin recovery is credited with saving the lives of two pilots, who resorted to it in 1991 to recover from a flat spin.

There is no indication when the new Tomahawk flight-test program will begin. By law, the FAA has 90 days to respond to the NTSB recommendations.

Piper responded to the NTSB with a brief statement in which it expressed strong support for the Tomahawk, and promised to work with the FAA in addressing the issues raised by the safety board.

In the statement, Piper called the Tomahawk "safe, airworthy and reliable when flown in accordance with the official pilots operating handbook."

Meanwhile, FAA officials refused requests from GANews & Flyer for an explanation of how the Tomahawk won a type certificate without evidence that it had completed all the tests required for certification.

That omission raises the question of what actions the FAA might require if the Tomahawk fails to meet a certification standard in the upcoming round of tests.

Would the Tomahawk continue to retain its type certificate? Or would the FAA rescind certification - and perhaps ground the fleet - pending a fix and retesting?

Agency officials declined to answer those questions - and others regarding the NTSB recommendations - on the grounds that it would be "inappropriate" to do so before the FAA responds to the NTSB.

Piper's own report on the Tomahawk's certification flight tests indicates that only turning flight stalls with flaps retracted were demonstrated.

The regulations also require testing of turning flight stalls with flaps extended, and accelerated stalls in both flap configurations. There is no documentation in Piper's report, however, that those stall tests were ever done.

In calling for completion of the full series of required stall tests, the NTSB said it is "concerned that production PA-38-112s may have stall characteristics different from those documented on the single pre-production airplane used during the original certification program."

To make sure, the board asked the FAA to use at least two airplanes in its tests, and to document any changes necessary "to bring these test airplanes into conformance with the type certificate."

The NTSB also asked the FAA to:

* Perform a series of wings-level stall tests "to ensure that the Tomahawk's stall is defined by a downward pitching motion of the airplane."

* Perform stall warning tests "to ensure that the stall warning horn activates at least five knots prior to the stall."

* Perform spin tests "to ensure that it is impossible to obtain unrecoverable spins with any use of the flight controls or throttle;" and to verify that the results obtained in the original certification program can be duplicated on production airplanes.

During the original certification program, Piper reported that "irrespective of the loading or entry or number of turns, the (Tomahawk) will recover (from a spin) in one additional turn after input of anti-spin controls."

The NTSB expressed doubts about that claim, however, in light of several reports of Tomahawks entering dangerous flat spins from a normal spin entry.

The NTSB asked for the wings-level and stall warning tests after learning about the results of a 1979 Swedish National Aeronautics Board investigation of the Tomahawk's stall/spin characteristics.

After performing more than 60 stalls with two production Tomahawks - one with two stall strips installed and the other with four - the Swedes concluded that the airplane did not meet FAA certification requirements for wings-level stall characteristics, or the FAA requirement for a stall warning.

FAA certification regulations require that a wings-level stall be characterized by a downward pitching motion. The Swedish tests, however, found that Tomahawk stalls were characterized by a roll disturbance, but no pitch change.

Additionally, the Swedish tests found that, contrary to certification requirements, the Tomahawk's stall warning horn "is engaged too late, approximately two knots prior to stalling with full flap, and three to six knots prior to stall without flap."

Although the Swedish tests were done in 1979, only a year after the Tomahawk debuted, the NTSB said it only recently was made aware of them.

Piper delivered nearly 2,500 Tomahawks between 1978 and 1982, when production ceased. Approximately 1,100 remain on the active U.S. register.

Since the two-seat trainer was introduced, it has been involved in 51 U.S. stall-spin accidents that resulted in 49 deaths. It also has been the subject of numerous pilot reports regarding its unpredictable stall-spin characteristics and tendency to enter flat spins.

A recent report by the AOPA Air Safety Foundation found that the Tomahawk stall/spin accident rate was nearly double that of comparable training aircraft such as the Cessna 150/152 , Beech Skipper and Grumman AA1.

The NTSB's own comparison between the Tomahawk and the Cessna 150/152 found the Tomahawk's fatal stall/spin accident rate was from three to five times higher than the Cessna trainer.

The NTSB noted that at least seven of the Tomahawk stall/spin accidents that it has investigated "involved inadvertent spins that occurred during instructional flights while attempting slow flight or stall training."

Piper developed and tested the Tomahawk in Vero Beach, Florida, but moved production of the airplane to Lock Haven, Pennsylvania, after certification was granted.

Only a year after introducing the airplane, Piper added two additional stall strips to Tomahawk wings (the original wing design included two stall strips) in an attempt to improve the airplane's stall characteristics.

Stall strips are triangular cross-section blocks approximately eight inches long and one-half-inch wide, placed on the outboard leading edge of the wing. They are designed to modify the stall characteristics of an airplane by inducting a stall at a controlled location along the wing.

Although the FAA concedes that it never evaluated the stall characteristics of the production Tomahawk in either the two-strip or four-strip stall configuration, it issued an AD in 1983 mandating that all existing airplanes be retrofitted with the additional stall strips.

According to Piper's own test pilots, however, that FAA-mandated fix offered little or no improvement.

According to the NTSB, one former test pilot who worked at the Lock Haven facility from 1978 to '84 told the board that the Tomahawks he flew were "totally unpredictable; one never knew in which direction they would roll off, or to what degree, as the result of a stall."

The NTSB also reported that a second former test pilot, who worked for Piper at Lock Haven for six years beginning in 1979, told safety investigators earlier this year that Tomahawks "were very unpredictable in a stall. Each airplane did not perform stalls the same from one flight to the other," the test pilot is quoted as saying.

A third former Piper test pilot, also interviewed by the NTSB earlier this year, reportedly told investigators that production Tomahawks "were nothing like the article certified (by the FAA) as far as stall characteristics are concerned."

According to the NTSB, this test pilot said the additional stall strips that Piper added to tame the airplane's stall characteristics "did not eliminate the stall/spin defects that he observed in the airplane."


 

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