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Tragic Effects of Wet Grass

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 aircraft’s 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 aircraft’s 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 Pilot’s 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 manufacturer’s approved take-off performance charts for conditions not covered by the manufacturer’s 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 manufacturer’s 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 manufacturer’s 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 aircraft’s 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.