Full Flap Takeoff on Wet Grass Maybe Not . . .
On July 27, 1998, a pilot and three passengers were departing in a Piper
PA 28 on a visual flight rules (VFR) flight from Espanola West to Ottawa, Ontario.
The runway surface was grass on uneven, sandy soil, made soft from recent rain. The
pilot made two excursions down the runway before the aircraft became airborne on the
third excursion. After the aircraft became airborne, it struck trees to the left of
the departure path and crashed into a wooded area. An intense fire immediately broke
out and consumed the aircraft cabin. One infant passenger perished in the aircraft
fire, while the pilot and the other two passengers escaped the burning aircraft but
died later from their burns. This synopsis is based on Transportation Safety Board
of Canada (TSB) Final Report A98O0190.
The pilot held a valid private pilot licence and was qualified for the flight. He
had approximately 350 hr. of total flying time. Visual meteorological conditions
(VMC) existed in the area at the time of the occurrence and the temperature was
23° C. It had been raining earlier in the day, and the pilot had delayed his
departure until the weather improved.
The runway, approximately 2900 ft. long, was surrounded by trees and was
oriented on a heading of 283° M. There were patches of ground on the runway
surface where the soil was very soft. The grass had been cut recently and was two to
three inches long. It was learned from an operator familiar with the airport that
they did not permit their tricycle-equipped aircraft to use the airstrip after rain
had fallen because the soil becomes very soft when wet. Higher ground is located at
the end of the departure runway and to the west.
Witnesses reported that the aircrafts engine seemed to be developing
considerable power and that they did not note any change in the sound of the engine
until after the aircraft struck the trees. The aircrafts flight after liftoff
was described as floating and hovering. The nose of the aircraft was then observed to
lower, and the aircraft started to bank to the left just before it struck the first
tree. An intense fuel-fed fire erupted on or immediately after impact.
The wreckage was examined at the site. The propeller marks on the trees and the
condition of the propeller showed that the engine was producing high power. Although
much of the aircraft was burned away, durable materials, such as hinges, steel cables,
and heavy metal, remained. All the aircraft flight control surfaces were accounted for,
and the control cables were intact at the time of the crash. The flaps were completely
burned away; however, the flap control handle was found locked in the 40° position
(full flaps).
The maximum allowable take-off weight for the aircraft was 2325 lb. The TSB
was unable to determine the exact weight at takeoff because the baggage was never
weighed and the exact fuel quantity is not known. However, the weight at takeoff was
estimated to be between 2300 and 2400 lb.
The Pilots Operating Manual (POM) for the aircraft contains
performance figures for takeoff from a paved, level, dry runway at the maximum gross
take-off weight of 2325 lb. Using a temperature of 23° C and the preceding
conditions, the take-off run was calculated to be 1255 ft. using no flaps and
965 ft. using 25° of flap. The take-off distance to clear a 50-ft. obstacle at
the end of the runway, using 25° of flap, was 1760 ft. A Transport Canada
brochure, entitled Light Aircraft Operating Tips (TP 4441E), provides
supplementary information to a manufacturers approved take-off performance charts
for conditions not covered by the manufacturers tests. The publication suggests
that the take-off ground roll should be increased by 10% for a runway surface that is
rough, rocky, or covered with short grass (up to four inches). It further suggests
that the ground roll should be increased by 75% or more for a runway with a soft
surface (mud, snow, etc.). With the flaps set at 25°, the combined penalties would
result in a required take-off ground roll of at least 1858 ft. and, to clear a
50-ft. obstacle, at least 2653 ft. There are no take-off performance charts
available for the aircraft if it is operated above the maximum gross take-off weight.
The POM notes that takeoffs are normally made with the flaps up; however, for
short field takeoffs and for takeoffs under difficult conditions, such as deep grass
or a soft surface, take-off distances can be reduced appreciably by lowering the flaps
to 25° and rotating at lower airspeeds. However, the POM does not recommend nor contain
any performance charts for takeoffs with full flaps. Extending some flap during takeoff
will generally result in a shorter take-off run and a better angle of climb; however,
using full flaps results in a low ratio of lift to induced drag and a reduced the climb
angle. When effectively performed, the soft field take-off technique will result in a
shorter take-off ground roll; however, any attempt to force the aircraft into the air
prematurely results in an increased take-off distance and a degraded climb
performance.
Analysis The winds were generally from 270° to
300° at 10 to 15 kt with gusts. As the runway take-off direction was 283°, it
is unlikely that the aircraft was greatly affected by the wind conditions and the
local topography, except that a headwind would have increased aircraft take-off
performance.
Based on the three excursions down the runway, the witnesses descriptions
of the engine noise, and the examination of the engine, it was concluded that the
engine was producing the required power. There was nothing found in the wreckage to
indicate that there was any aircraft malfunction before the crash.
The aircraft was at or near the maximum allowable weight and would, therefore,
require the maximum calculated take-off distance and possibly more. The runway surface
conditions and the fully extended flaps further increased the take-off distance and the
distance required to climb to an altitude to safely pass over the trees. The first
excursion down the runway may have been made by the pilot in an
attempt to establish runway surface conditions and that the second and third excursions
may have been take-off attempts. Based on witness accounts and the fact that a pilot
would normally change some parameter of the aircraft configuration after experiencing
a failed take-off attempt, if the first excursion down the runway was a take-off
attempt, it is probable that the first attempt was conducted without any flaps extended
and the second attempt with the flaps extended to 25°, the manufacturers
recommended flap extension for a soft-field takeoff. It was concluded that the flaps
were set at 40° during the last take-off attempt because of the manner in which the
flaps are operated and the lever locked in position. Takeoffs with flaps extended fully
are not a recommended practice, but the aircraft did become airborne. However, with the
flaps fully extended, the high drag resulted in a loss of climb performance, which made
it impossible for the aircraft to climb fast enough to clear the trees at the end of
the runway. In an attempt to clear the trees, the pilot probably raised the nose of the
aircraft, but because of the low speed and high drag, the aircraft stalled.
Corrections can be made to published take-off distance estimates using published
information; however, there is no manufacturers published take-off performance
information available for the conditions of the occurrence flight. It is not possible,
therefore, to estimate the take-off run required or the distance required to reach
50 ft. above ground at the end of the runway for an aircraft above the maximum
certificated take-off weight with full flaps extended. The distance required to clear
a 50-ft. obstacle at the end of the runway with full flaps extended would undoubtedly
be more than it would with the flaps set to 25°.
The impact was survivable, most probably because the impacts with the trees and
the angles at which they were struck absorbed most of the aircrafts momentum
and because the passengers were wearing their restraints; however, the fire that
followed the impact led to the fatalities.
In the end, the TSB determined that the pilot attempted to take off in conditions
where a successful takeoff could not be made; the conditions being the high aircraft
weight and the soft, grassy runway. The fully extended flaps contributed to the
occurrence when they prevented the aircraft from climbing quickly enough to safely
pass above trees at the end of the runway after the aircraft became airborne.
This tragic and preventable occurrence should serve as a lesson for all of us who
will fly from similarly short, unimproved strips in the coming spring and summer. To
obtain a copy of TP 4441E, contact your
regional Transport Canada office or the editor of the ASL.
|