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 takeoff. 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 takeoff. 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 takeoff. 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 takeoff indicated that the wind was a gusty crosswind. The
average wind direction and speed was 325 degrees true at 19 kts.
However, during the takeoff, 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 takeoff 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 takeoff 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 takeoff 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:
Takeoff 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 takeoff in gusty or strong crosswind conditions, maximum
takeoff thrust is recommended. This maximizes available runway and
minimizes the airplane exposure to gusty conditions during the
rotation and takeoff 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.
