Incident Overview
The de Havilland DHC-8 aircraft (registration C-FGRP, serial number 207), operated by Jazz Air Inc. as flight JZA7841, departed Kingston, Ontario, at 1403 eastern standard time for Toronto/Lester B. Pearson International Airport. The first officer was the pilot flying. During initial climb, the first officer noted that abnormal forward pressure on the control column was required to prevent the aircraft from pitching nose up. To counter the pitch-up, he trimmed the aircraft nose down. Thirty seconds after becoming airborne, at approximately 350 feet above ground level (about 700 feet asl), the first officer had applied full nose-down trim. The required forward pressure increased as the aircraft accelerated, and the first officer requested the captain's assistance in holding the control column forward. The captain declared an emergency, citing control difficulties and the possibility of landing at Trenton, Ontario. The crew levelled the aircraft at 4000 feet asl and pulled the elevator pitch disconnect handle, isolating the left and right elevators. The captain's elevator control functioned normally after the disconnect, and he decided to continue the flight. The crew conducted a flapless landing at Toronto without further incident.
History of the Flight
The flight crew had spent the night in Kingston and were waiting when the aircraft arrived. This was their first flight of the day; the aircraft had already flown four prior flights with three different crews. The captain spoke with the incoming captain, while the first officer performed the preflight inspection. No flight control anomalies were reported on the previous flight, and the first officer saw nothing amiss. The turn-around time was about 15 minutes. During the take-off run at approximately 1430 eastern daylight time, the controls felt lighter than normal, and at rotation almost no nose-up force was required. After becoming airborne, increasing forward pressure was needed to maintain climb attitude, even with full nose-down trim.
Pitch Control System and Spring Tab Operation
The DHC-8 pitch control system comprises two separate elevator control cable circuits, each operating an independently mounted, spring tab-assisted elevator. Each elevator has a spring tab at the inboard end and a trim tab at the outboard end. The left elevator is actuated by the captain's control column via the left cable circuit and left elevator spring tab; the right elevator is similarly operated by the first officer's controls. The two control columns are normally interconnected but can be disconnected by a pitch disconnect system if one circuit jams.
The spring tab system includes two mass balances extending forward of each spring tab leading edge. Each mass balance assembly consists of two weights bolted together and secured to the arm with two bolts.
Post-Inspection Findings
After landing, inspection revealed that half of one balance weight from the right-side elevator spring tab, along with the two nuts that secure it, were missing. The two bolts had migrated out of the remaining half weight and jammed on the top surface of the elevator, holding the elevator spring tab in the trailing-edge-down position.
The balancing and independent inspection of the spring tab had been performed approximately one month before the occurrence. Specific details were unavailable, but the top of the jammed weight had been ground down, indicating it had been removed for balancing. After adjustment, the weights would have been reinstalled and the tab rechecked for balance. It is possible that the nuts and washers were not reinstalled for the balance check, or were left loose to facilitate further removal, and the final tightening step was overlooked. Without the nuts, the bolts migrated out; when the outboard weight fell off, the bolts jammed.
The balancing task was completed late at night by an AME who had travelled from Halifax and worked a long day. The independent inspection the next morning did not detect the loose nuts; no explanation was found.
Probable Cause
The nuts securing the counter balance weight to the weight arm were not tightened. They fell off, which in turn allowed the bolts to migrate out of the weights. When the outboard weight fell off, the bolts jammed on the top surface of the elevator. The independent inspection was not adequate, in that it did not reveal that the securing nuts were not tight.