Aug. 19, 2017
The pilot/builder was approaching the airport for landing on a 3-mile left base leg at the conclusion of a cross-country flight. Shortly after being cleared for landing, witnesses observed the airplane make a steep left turn and dive toward the terrain. The airplane hit the ground in a near-vertical, nose-low attitude; the wreckage was consumed by a postcrash fire.
Examination revealed no mechanical malfunctions or anomalies that would have precluded normal operation of the airplane or engine.
The airplane was equipped with a “cruciform” (mid-mounted horizontal stabilizer) tail. Several years before the pilot completed the airplane, the kit manufacturer disseminated a report to owners regarding the design and aerodynamic characteristics of the cruciform tail. It stated that the airplane’s tail “may stall on approach and turbulent conditions or upon crossing another airplane’s wake (or its own wake in a turn)” and that “rapid action must be taken to avoid diving straight into the ground.”
Where is the explanation why a cruciform configuration might be worse than a conventional, low tail position or a T-tail? The one similar case that comes to mind is the Grob SPn business jet, which crashed during testing. “On 29 November 2006, the second prototype was destroyed by a crash that resulted in the death of chief test pilot Gérard Guillaumaud; it had been performing a demonstration flight at the time of the accident. A subsequent investigation determined the primary cause of the accident was due to flutter in the elevators and tailplane.” Fundamentally, horizontal tailplane placement is only half of the story. “Unblanked” area of the vertical stabilizer and rudder are also important to maintain control at low speeds. For this reason, many light aircraft have the horizontal tail behind or in front of the vertical tail surfaces. T tails suffer as the horizontal tail may be blanked by the wake of the main wing when in deep stall. Other than on sailplanes, T-tails have become less common on light aircraft as a result.
In 1997, the CAFE Foundation published an Aircraft Performance Report on the Wheeler. I suspect this was the report referenced in the NTSB documentation. The report included a “Problem Areas” section which said, in part,
“LOW DIRECTIONAL STABILITY: The cruciform tail (mid-mounted horizontal stabilizer), wide tapering fuselage, and highly swept vertical stabilizer combine to produce low yaw stability. This was evident during dynamic testing, high angles of attack, and during excursions into high yaw situations at one g. An uncommanded right yaw with inadequate elevator control occured after 1 g clean stalls and persisted until airspeed over 85 kts had been regained. This behavior should receive more investigation by a professional test pilot wearing a parachute. While holding constant heading in uncoordinated flight at 120 kts panel airspeed, the nose would gradually pitch down as more yaw was introduced.Before the limits of the rudder were reached the elevator authority became insufficient to hold a level attitude. The airplane would stay highly yawed regardless of the rudder position commanded by the pilot as the nose increasingly pitched down. Once the airspeed increased to about 130 kts the airplane could be returned to controlled straight and level flight.”
Online search for “cafe wheeler performance review” should produce the full report.
Given the sailplane’s very low ground clearance, a T-tail is much less worrisome during an off-field landing where a conventional tail is subject to damage from vegetation.