Structural failure of elevator on Yak 52 during flight near Raydon

Casualties unknown • In-flight near Raydon Airfield, Suffolk, GB

A Yak 52 experienced a sudden pitch-down and control stick vibration after a portion of the right elevator detached during a private flight in Suffolk.

What happened

On 17 November 1998, a Yak 52, registration G-BXJB, was performing a private flight near Raydon Airfield, Suffolk. Following the completion of an aerobatic sequence with one passenger on board, the pilot began a descent at 350 kph to join the landing circuit. While at an altitude of roughly 1500 feet, the control stick suddenly moved forward and the aircraft entered a nose-down pitch, accompanied by intense vibrations through the stick. A loud noise was heard from the rear of the aircraft shortly after.

The pilot responded by reducing throttle and managed to stabilize the aircraft in level flight at approximately 400 feet. After declaring an emergency, the aircraft returned to its departure airfield and landed without further incident. During the flight, the passenger noted that a piece of the right elevator had broken away.

The investigation

Investigators examined the aluminum alloy framework of the failed elevator, which consists of a tubular spar and five ribs covered in fabric. The inspection of the wreckage revealed that cotton fabric had been used for the covering and that mold was present on the fabric, particularly near the tip. This mold was also found on other surfaces of the aircraft.

Analysis of the fabric showed eleven chordwise tears on the upper surface, though only one tear was located at a rib position. The edges of the upper and lower fabric had become frayed due to flapping in the slipstream. The investigation found significant structural damage to the ribs: the flanges on ribs 1 and 2 had fractured, and the webs of rib 1 showed evidence of very low cycle fatigue. Ribs 3 and 4 also exhibited similar fracture patterns. Additionally, the tip rib was crushed, and a fatigue crack was identified in the lower skin of the fairing.

Findings

  • The structural failure began with a chordwise tear in the upper surface fabric at the position of the second rib.
  • This initial tear was caused by the in-service degradation of the cotton fabric.
  • The failure of the aluminum ribs was driven by a mechanism of fabric 'ballooning' under aerodynamic loads, leading to low cycle fatigue.
  • The use of non-specified cotton fabric and the presence of mold contributed to the degradation of the control surface.

Probable cause

The break-up of the right elevator was initiated by a tear in the upper surface fabric caused by the degradation of the cotton covering, which subsequently led to structural fatigue of the elevator ribs.

Frequently asked questions

What happened in the 1998-11-17 YAK-52 accident near In-flight near Raydon Airfield, Suffolk, GB?

A Yak 52 experienced a sudden pitch-down and control stick vibration after a portion of the right elevator detached during a private flight in Suffolk.

What aircraft was involved and where did it happen?

The accident on 1998-11-17 involved a YAK-52, registration G-BXJB, at In-flight near Raydon Airfield, Suffolk, GB.

What was the probable cause of the accident?

The break-up of the right elevator was initiated by a tear in the upper surface fabric caused by the degradation of the cotton covering, which subsequently led to structural fatigue of the elevator ribs.

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