Flight History
On 29 June 2004, at 1650 UTC, a Grob G115E Tutor (registration G-BYXJ) was conducting a training flight near Salisbury, Wiltshire. The aircraft, operated by the Royal Air Force, had two crew members on board (instructor and student). The weather was fine with a surface wind of 210°/12 kt, visibility of 35 km, few clouds at 2,300 ft, and broken cloud at 25,000 ft.
After climbing to Flight Level 50 with high RPM and full power, the instructor performed HASELL checks, set 2,500 RPM, and confirmed the auxiliary fuel pump was off and the fuel was balanced. The aircraft then performed a clearing wing over to the right, followed by a loop at 130 kt. After a brief level flight, it entered a half Cuban eight, climbing at 130 kt with a 60° nose-up attitude. When the airspeed dropped below 100 kt, full back pressure on the control column and full left rudder were applied, resulting in a snap roll left through 180°. The aircraft was stabilized inverted in the climb before being pulled through to the horizon.
As the aircraft leveled at approximately 120 kt with full power, a loud bang occurred accompanied by extreme vibration. The instructor observed debris passing the canopy and felt airflow entering the cockpit. Suspecting a mid-air collision, he transmitted a Mayday call and noticed the canopy moving rearward and then detaching; neither occupant had touched the canopy latching system. The instructor saw that the propeller blades were damaged. After the engine was shut down, the vibration increased but then ceased. With the propeller stationary, it was evident that one blade was missing and another was severely damaged. Oil was visible on the right windscreen.
The instructor initially considered abandoning the aircraft but, after the vibration stopped, decided to perform a forced landing. Familiar with the area, he identified a suitable field previously used in practice. After passing through his planned high-key position, he completed forced landing checks and informed ATC of his position and intention to land. The aircraft touched down at approximately 60 kt in a standing crop. On the ground, the nose yawed right while the aircraft slid left, causing the left main landing gear to collapse. The aircraft came to rest after a ground roll of about 50 meters. Both crew members were uninjured and vacated the aircraft without difficulty. Subsequently, a coastguard helicopter transported them to Salisbury hospital for medical checks.
Aftermath and Examination
The AAIB examined the aircraft at the accident site. The Number 1 propeller blade had detached due to a failure of the hub. The blade was located approximately 3.5 km from the aircraft, with the fractured part of the hub still attached. Examination of the fracture surfaces indicated that the aluminum hub had failed due to high-cycle fatigue cracking originating at the front and back of the blade arm. This was later confirmed by initial metallurgical analysis.
As the blade separated, it struck the following blade, causing a large portion of that blade to detach. The out-of-balance forces generated before engine shutdown caused the canopy to detach and resulted in significant damage to the engine and its mountings: the left-hand upper engine mounting lug was severed from the crankcase, the engine mounting frame cracked and severely distorted, the left-hand magneto was pulled from its mounting, and the exhaust mufflers separated from the exhaust down pipes. The left-hand main landing gear and nose gear were extensively damaged during the forced landing.
At the time of the accident, the propeller (part number HO-V343K-V/183GY, serial number G22AB) had accumulated approximately 1,710 hours since manufacture in May 1999 and about 249 hours since its previous overhaul in August 2003. The investigation into the cause of the propeller hub failure is continuing.
Safety Actions
Immediately after the accident, operations of the Grob 115E Tutor aircraft ceased. The propeller manufacturer, Hoffmann Propeller GmbH & Co KG, issued Service Bulletin 61-10-03 SB E 15, requiring eddy current non-destructive testing of HO-V343 propeller hubs to detect cracks in the blade arms. The bulletin also requires checks for blade looseness ('blade shake') and verification of blade retention nut torque values. The procedure is described in Hoffmann Service Instruction 61-10-05 SI E 4. This service bulletin has been mandated by the German Luftfahrt Bundesamt (Civil Aviation Authority) under EASA-approved Airworthiness Directive LTA D-2004-352R2.