On Tuesday, October 3rd at approximately 6:45pm, N8360K, a 1981 Piper PA-44 Seminole, operated by Hillsboro Aero Academy, tragically crashed killing two and leaving a third in critical condition. Our hearts go out to the victims and their families.
Video footage of the moments before the aircraft crashed into a home in Newberg, Oregon, show the in-tact aircraft descending in an uncontrolled vertical spin.
Reports indicate that the occupants onboard the aircraft included a student pilot and a Hillsboro Aero Academy flight instructor. Both the instructor and student were killed in the crash. The rear-seat passenger was rescued from the wreckage and airlifted for emergency surgery. No one on the ground was injured in the crash and there was no post-crash fire.
Authorities have not yet released any formal statement regarding the cause of the crash. Aviation experts who have reviewed the video footage, however, have speculated that the crash may have been caused a wide spectrum of possibilities including a catastrophic control system failure to operator error resulting in an unrecoverable stall/spin.
The Piper PA-44 Seminole is a 4-seat, light twin-engine aircraft, commonly used for flight training. The aircraft is typically equipped with two air-cooled 180hp Lycoming 4-cylinder counter-rotating engines and the aircraft features a T-tail.
Vmc demonstration maneuver
This article will focus on one possible cause of such spins in twin-engine T-tailed aircraft, such as the Seminole: A critical pilot error during a Vmc demonstration. While it is early in this accident investigation, we are not suggesting that this was the cause of the accident.
Vmc is the minimum speed a twin-engine aircraft can be flown on one engine and still maintain directional control. V means speed and mc means minimum controllable. With one engine inoperative, the asymmetrical thrust of the operating engine must be balanced by the rudder. As the aircraft’s speed decreases, so does rudder authority, until eventually, the aircraft will roll into the dead (not operating) engine even with full rudder deflection.
For pilots, part of obtaining a multi-engine airplane rating involves the student learning to successfully identify, and recover from, a loss of control of the aircraft at the Vmc transition. This critical maneuver is important to be able to execute, but also carries with it some inherent risks. If a twin-engine aircraft with only one engine operating is allowed to stall, the aircraft has a tendency to want to roll over.
An aircraft “stalls” when the airflow over one or both wings stop producing lift due to too high an angle of attack. Pitch an aircraft up, and it will climb, until the wing can provide no more lift, at which point the wing stalls, causing the wing, and aircraft, to drop. A spin occurs when only one wing is stalled, and the aircraft rotates in a spiral as it drops.
Because a twin-engine aircraft performing a Vmc demo has one engine at full power, that wing is much more difficult to stall due to the additional airflow provided by the propeller. If the wing on the dead engine stalls, the aircraft rolls. This rolling motion, coupled with a stalled wing, creates a spin. In a normal Vmc recovery, the procedure is to pitch the nose down, reduce power on the operating engine, and level the wings before a stall occurs. If a stall inadvertently does occur, the situation can become dire quickly. The weight of the engines out on the wings add momentum to the roll, making a spin recovery more difficult.
Then there is the T-tail, which can become ineffective at stall due to the turbulent wash off of the stalled wing, blocking airflow over the tail. T-tails are designed to put the elevator up high in clean air undisturbed by the wings. This is the case in normal situations and is a benefit to elevator effectivity, but during a stall, the angle of the aircraft can put the elevator into the turbulent air produced by the stalled wing. This situation is called a “deep stall” which is particularly perilous because the elevators are normally used to pitch down and break the stall. If the elevators don’t have sufficient airflow, the stall can require extreme altitude to recover, or some situations may be unrecoverable. The footage of the accident aircraft is consistent with this situation, though it is not conclusive.
It is anticipated that the ensuing investigation will include a thorough examination of the accident Seminole’s control system. If no discrepancies are found, an inadvertent deep stall will certainly be a prime candidate in the search to explain this most tragic accident.
An Example of a T-Tailed Aircraft Experiencing Normal Airflow over the Tail (Top), and Then One in a “Deep Stall” where Airflow Over the Tail is Obstructed (Bottom).
