The Australian Transport Safety Bureau did not conduct an on-scene investigation of this occurrence.
FACTUAL INFORMATION
On 11 October 2004, a Boeing Company 737-86Q (737) aircraft, registered VH-VOF, was being operated on a scheduled passenger service from Perth, WA, to Sydney, NSW. The copilot was the handling pilot for the flight.
At 1124 western standard time, as the aircraft became airborne from runway 03 at Perth, the cabin crew members seated at the rear of the aircraft felt and heard the aft fuselage scrape the runway. At about FL160 during the climb, the cabin crew alerted the flight crew of a possible tail strike during the take-off. The pilot in command assumed control of the aircraft and elected to return to Perth. The aircraft was descended to 9,000 ft, and during the descent the flight crew performed the 737 Quick Reference Handbook (QRH) Non-Normal Checklist for a tail strike on take-off. However, because no cabin pressurisation problems had occurred following the suspected tail strike, they elected to leave the aircraft pressurised.
Air traffic control cleared the crew to hold the aircraft to the west of Perth to allow sufficient fuel burn to reduce the aircraft to its maximum permitted landing weight, and the aircraft landed at Perth about 2 hours later.
Engineering inspection confirmed that the aircraft had sustained a tail strike during the take-off. The tail strike ground contact was slight, and resulted in minor scuffing to the base of the shoe on the tail-skid assembly. The crushable cartridge within the tail-skid assembly was undamaged.
The load trim sheet provided to the flight crew by the operator indicated that the planned take-off weight was 71,331 kg, which was used by the crew in conjunction with data from the operator's Airport Analysis Manual to determine the take-off speeds for the departure from Perth, using the full length of runway 03. Those speeds were the take-off decision speed (V1) of 142 kts, the take-off rotation speed (VR) of 147 kts, and the take-off safety speed (V2) of 151 kts.
Following the occurrence, all luggage and freight carried in the cargo compartments of the aircraft was re-weighed. The total cargo compartment load noted on the original load and trim sheet provided to the crew was about 160 kg less than the actual weight revealed by the re-weigh. The actual take-off weight was therefore 71,493 kg, and the take-off speeds for that weight were only 1 kt greater than the speeds determined by the flight crew. The minor discrepancy between the planned and actual weights was not a factor in the occurrence, and the aircraft was within its approved centre of gravity limits.
Perth aerodrome automatic terminal information service (ATIS) provided current, routine information to arriving and departing aircraft at Perth by means of continuous and repetitive radio broadcasts. Information Papa was current at the time of the occurrence and advised that the duty runway was runway 03 (wet), wind was 320 degrees magnetic at 20 kts, with an associated crosswind of 18 kts. The barometric pressure was 1010 hPa, and the temperature was 20 degrees C. The ATIS also included information that windshear was present in the vicinity of the aerodrome, and that the wind direction and speed at a height of 250 ft above ground level was 330 degrees magnetic at 25 kts, gusting to 35 kts.
The Bureau of Meteorology one-minute wind data at the time of the take-off indicated that the wind was a gusty crosswind. The average wind direction and speed was 325 degrees true at 19 kts. However, during the take-off, the wind direction and speed fluctuated between 319 and 331 degrees true, and from 18 to 21 kts.
The aircraft was fitted with an Allied Signal solid state flight data recorder. The ATSB analysed the recorded flight data to assist in establishing the factors that led to the tail strike.
The computed airspeed data revealed that acceleration was normal up to V1, at which point the aircraft's speed remained constant at 142 kts until rotation was initiated. At lift-off, the computed airspeed was about 152 kts. During the period between the commencement of rotation and the lift off of the main landing gears, the aircraft's nose-up pitch increased steadily from 0 degrees to 13.2 degrees. There was a 1.19g spike in the normal load factor data at the point of lift off.
About 23 degrees of left control wheel was applied throughout the take-off run until the aircraft was rotated. Left control wheel input increased from the point of rotation, and was about 43 degrees when the main landing gears became airborne. The left aileron was displaced 9.7 degrees up, and flight spoiler panels 3 and 4 were deployed 4.4 degrees and 11 degrees respectively at lift off. During the 2 seconds following lift off, left control wheel input increased to 48.8 degrees, then reduced to 28.7 degrees. The deployment of flight spoiler panels 3 and 4 increased to 13.9 degrees and 13.5 degrees respectively with the application of 48.8 degrees of left control wheel deflection. Right rudder was also applied during the take-off roll, which was consistent with the prevailing crosswind conditions.
Aircraft can achieve high angles of pitch relative to a runway during both take-off and landing segments of flight. If the pitch angle exceeds prescribed limits when an aircraft is close to the ground, the aft underside portion of the aircraft may contact the runway surface. That contact is referred to as a tail strike, and can result in significant damage to the aircraft. Aircraft manufacturers provide guidance to flight crews on the correct pitch rates and speeds to avoid tail strikes during take-off and landing manoeuvres.
The point of minimum tail clearance during take-off occurs after the lift-off speed has been attained. Initiation of rotation before the scheduled VR speed or rotation at an excessive rate will reduce the minimum tail clearance, and under those circumstances, contact with the ground will probably occur.
Chapter 3 of the operator's 737 Flight Crew Training Manual (FCTM) provided the following information on the take-off rotation manoeuvre:
When a smooth continuous rotation is initiated at VR, tail clearance margin is assured because computed takeoff speeds depicted in the QRH, airport analysis, or FMC, are adjusted to provide adequate tail clearance.
The FCTM also provided information that:
Take-off and initial climb performance depend on rotating at the correct airspeed and proper rate to the rotation target attitude. Early or rapid rotation may cause a tail strike.
The FCTM provided information that the lift-off attitude of the 737-800 was 8.2 degrees with the wing flaps extended 5 degrees. The minimum tail clearance at that pitch attitude was 51 cm, and the tail strike pitch attitude was 11.0 degrees with the main landing gear oleo struts extended.
The FCTM also provided information and recommendations on flight manoeuvres and techniques in gusty wind and strong wind conditions for the 737, as follows:
For take-off in gusty or strong crosswind conditions, maximum take-off thrust is recommended. This maximizes available runway and minimizes the airplane exposure to gusty conditions during the rotation and take-off maneuver [sic].
Avoid rotation during a gust. If a gust is experienced near VR, as indicated by stagnant airspeed or rapid airspeed acceleration, momentarily delay rotation. This slight delay allows the airplane additional time to accelerate through the gust and the resulting additional airspeed improves the tail clearance margin. Do not rotate early or use a higher than normal rotation rate in an attempt to clear the ground and reduce the gust effect because this reduces tail clearance margins. Limit control wheel input to that required to keep the wings level. Use of excessive control wheel may cause spoilers to rise which has the effect of reducing tail clearance limits. All of these factors provide maximum energy to accelerate through gusts while maintaining tail clearance margins at liftoff.
The aircraft manufacturer advised that deflection of flight spoilers and ailerons due to control wheel input reduces the lift coefficient of the wing and that the reduction in lift coefficient effectively reduces the tail clearance at rotation during the take-off manoeuvre.