CAB Reports Findings in Crash of Mooney Model 18 Lightplane


On Sept. 7, 1959, about 1430 EDT, a Mooney M-l8C55, N4174, crashed 1 mi. west of the Culmerville Airport near Butler, Pa. Donald W. Bailey, the pilot and sole occupant, received fatal injuries. The airplane, a single-engine, single-place, low-wing monoplane, was demolished.

Mr. Bailey planned a local VFR flight of 20 min. duration. He was seen to take off in N4174 and climb to about 1,500 ft. northwest of the airport where he executed several "lazy-eight" maneuvers. At approximately 1430 several persons on the ground saw the aircraft in straight and level flight on a southerly heading. A loud crack was heard and parts were seen to separate from the aircraft. Immediately thereafter N4174 entered a spin and crashed.

The Board concludes that this accident was caused by the inflight separation of the trailing edge member of the right horizontal stabilizer. Investigation revealed very poor bonding of the glued wood joints of this structure due to improper production techniques. In addition, evidence was found throughout the aircraft of poor design and production practices in both welded metal parts and glued wood joints.

As a result of this investigation and the Board's recommendations, an Airworthiness Directive was issued by the administrator of the Federal Aviation Agency.

Investigation

Mr. Bailey possessed a currently effective FAA commercial pilot certificate with a rating for single-engine land airplanes and an instrument rating. He had accumulated about 900 flying hours. Mr. Bailey's total flight experience in N4174 was 1 hr. 40 min.

The flight on which the crash occurred was a proficiency flight in the local area and was to be of 20 min. duration. The weather conditions were: ceiling and visibility unlimited. The surface wind was from the west-southwest at 5 mph.

Mr. Bailey took off on the west runway about 1415. He circled the airport climbing to an altitude of about 1,200 - 1,500 ft. above ground. About 1-1 mi. northwest of the airport he was observed executing "lazy-eight" maneuvers.

At approximately 1430, several persons on the ground saw the Mooney aircraft. They stated that it was proceeding in a southerly direction in straight and level flight. Several stated that they heard a loud sharp crack and saw objects separate from the aircraft. Several others heard the sound and then observed the aircraft in a spin with pieces falling behind it. Immediately thereafter the aircraft crashed 1 mi. west of the Culmerville Airport. There was no fire.

Structural Failure

Several aircraft parts were found back along the flight path and confirmed the observations that inflight structural failure had occurred. The right side of the horizontal stabilizer was found 2,600 ft. northwest of the main wreckage site; the right elevator was found 1,100 ft. northwest; and the fin and rudder, Plexiglas, and a piece of right wing plywood were found 550 ft. northwest of the wreckage.

The main portion of the aircraft struck the ground in a near-vertical descent. Impact damage indicated it was pitched nose down to a 30 or 40 degree angle and yawed to the left. All components, other than those which separated in flight, were found within a 25-ft. radius of the powerplant.

The right wing spar had failed downward from inflight. overloads at two points; approximately 1 ft. outboard of the attach point and 4 ft. 9 in. outboard of the attach point. The section between the two breaks was in the main wreckage with the landing gear still attached. The right flap with its hinges had separated from the wing and was bent upward in a "U" shape which corresponded closely with the curvature of the top of the fuselage midsection. The hinge welds on the flap were cracked at impact. One piece of plywood, identified as a portion of the right wing lower surface at and forward of the spar just outboard of the inboard failure, was found 550 ft. from the wreckage. The size of this piece, which was originally glued to the spar, was 10.3 in. long but showed bonding of the glued joint for only about 3.5 in.

Inspection of the right horizontal stabilizer, which separated in flight, revealed that the spar had failed at the right attach bolt in downward overload. A section of the right trailing edge member extending from a point outboard of the inboard hinge to the No. 3 rib had pulled out. It remained attached to the elevator by the middle hinge when the elevator had separated from the stabilizer. The outboard elevator hinge remained with the stabilizer. The glued joint of the No. 2 rib and the trailing edge member had separated with only slight failure of the wood. The laminated plies of the middle hinge block were also separated with no wood failure. The rearmost lamination was cracked rearward by the hinge bolt. In addition, the No. 1 rib had separated from the spar with very little wood failure.

The right elevator was found 1,100 ft. northwest of the main wreckage area. It was bowed spanwise so that the middle hinge was about 2 in. lower than the root and tip. The middle hinge was intact with the portion of trailing edge member mentioned above attached. The inboard and outboard hinges had separated at the upper and lower welds. These welds had very little penetration.

The vertical stabilizer failed to the left and with the rudder attached fell 550 ft. from the wreckage. When the rudder separated from the aircraft the lower hinge failed in the weld. The rib trailing edge glued butt point adjacent to the middle hinge was loose. It was being held in place by one brad and the joint showed very little wood failure.

Impact Forces

All other damage to the aircraft was the result of impact forces. There was some evidence of deterioration present in several of the glued joints which had separated with very little wood failure. In other joints, which had failed in a similar manner, there was no evidence of any deterioration. (It was noted that none of the glued joints was reinforced with gusset plates but depended entirely upon the strength of the glue to hold them.)

In view of the findings during this investigation and the fact that the Model 18 production ceased in 1956, the Board carried its investigation into Mooney Aircraft, Inc.'s, present production methods for the Model 20A. The present company, which took over control of Mooney Aircraft, Inc., in 1956, maintains a strict inspection system to check all stock as it is received into the plant to ensure conformity to specifications. Quality control of all fabricated parts is maintained by visual inspection of each operation and comparison with blueprints. In addition, a conformity report on each lot of parts has to be filled out staling the number of units inspected, the workmanship, and conformance to the drawings. Further, each step in the assembly and subassembly must be inspected and signed off before the next operation can be started.

Analysis

The evidence indicates that the initial inflight failure occurred when the glue joint of the No. 2 rib separated allowing the trailing edge member of the right horizontal stabilizer to pull out. Airload bowed the elevator down in the middle, its inboard and outboard hinges remaining intact. As the elevator was bowed downward and the lateral distance between the inboard and outboard hinges decreased, an abnormally high down- ward loading was consequently imposed on the stabilizer, failing it downward at its attach point. It is probable that as the stabilizer separated from the aircraft the outboard elevator hinge failed and the elevator remained attached to the aircraft momentarily until the inboard hinge also failed. This is indicated by the location of the stabilizer farther from the wreckage than the elevator.

The immediate result of the loss of the stabilizer was a violent nosedown pitching of the aircraft, which failed the right wing spar downward in two places. Following this the aircraft rolled rapidly to the right and the vertical fin separated. The aircraft then fell nearly vertically to the ground.

The generally poor condition of glued wood joints throughout the aircraft appears to be a result of poor production techniques. Although deterioration from weathering was noted in some which had failed, it is not believed to have been of such a degree as to cause separation with little or no wood failure. The design practice of using glued butt joints without gussets or the equivalent is considered poor. In the later Models 20 and 20A this problem is overcome because the stabilizer and wings are completely covered with plywood and this surface acts as gusset for the structure.

Similarly, poor production techniques were noted in numerous welds throughout the aircraft. Because of poor penetration, several had failed in the weld deposit or at the juncture of the weld deposit and the parent metal. The weld should, if properly made, be the strongest point in a tubular member and loads exceeding design strength of the part should fail it at a point adjacent to the weld rather than through the weld itself.

Conclusions

The Board concludes that the inflight structural failure resulted from the separation of a glued wood joint. The Board further concludes that proper bonding between the wood and glue was not obtained during production because of improper techniques and control. It is also concluded that the design practices did not provide a sufficient margin of safety to guard against weakening of the structure over a prolonged period.

As a result of recommendations made by the Board to the administrator of the Federal Aviation Agency following this investigation, the latter issued an airworthiness directive to correct these deficiencies.

Probable Cause

The Board determines that the probable cause of this accident was an inflight structural failure brought about by the separation of an improperly glued wood joint.

By the Civil Aeronautics Board:
WHITNEY GILLILLAND Chairman
CHAN GURNEY Vice Chairman
C. JOSEPH MINETTI Member
ALAN S. BOYD Member

Supplementary Data

The Civil Aeronautics Board was notified of this accident shortly after it occurred on Sept. 7, 1959. An investigation was immediately initiated in accordance with Section 701 (a) (2) of the Federal Aviation Act of 1958. Depositions ordered by the Board were taken at the Culmerville Airport, Culmerville, Pa., on Oct. 13, 1959, and in Kerrville, Tex., on Oct. 15, 1959.

The Aircraft

N4174, a Mooney M-18C55, manufacturer's serial number 339, was manufactured July, 1955. A production test flight was performed on Apr. 30, 1956, and the aircraft found to be airworthy. It was classified in the standard category. A current certificate of airworthiness was issued Jan. 3, 1959. The current certificate of registration, dated Sept. 2, 1959, listed Fred 0. Eiler of Tarentum, Pa., as the owner. The aircraft and engine had a total of 356 hr., of which 6 hr. had been accumulated since the last 100-hr. inspection Aug. 26, 1959. It was equipped with a Continental engine, A65-12, and a Flottorp propeller, 65A66, installed new on May 21, 1959.

The Pilot

Donald W. Bailey, age 24, resided at Gibsana, Pa. He possessed a currently effective FAA commercial pilot certificate with an airplane single-engine land and an instrument rating. He had accumulated approximately 900 flying hours, of which 1 hr. 40 min. were in the Mooney M-18.

FAA Airworthiness Directive (AD 59-22-03)


Footnotes:

ALL TIMES herein are Eastern Daylight based on the 24-hr. clock. (Continue)

LAZY-EIGHT: an advanced flight training maneuver which combines the dive, the turn, and the climb. This maneuver does not impose excessive or abnormal loads on the aircraft when properly executed. (Continue)

This article, kindly provided to us by Dick Rank, N125C, is reprinted from AVIATION WEEK magazine, July 25, 1960 issue.


21 November 2000