How much gas we can carry with the bodies and gear we have on board and where are we going to put that gear in the aircraft?
We may be modifying, or have just modified, our ultralight or homebuilt and we need to think about what those modifications have done to the empty weight of the aircraft and the C of G.
The pilot of a Grumman American Cheetah AA5A lost control on take-off and slammed into a house killing himself and a passenger.The aircraft was on a cross country flight from Texas to Alaska and had just departed after a planned refueling stop where the pilot had taken on 36 gallons of avgas.
Witnesses reported that on the take-off roll, the nose gear lifted off very late and the aircraft did not become airborne until it was within 500 feet of the end of the 4,380-foot runway. The aircraft then entered a steep climb attitude until about 100 feet where it started a shallow turn to the left and then departed controlled flight. The departure was on runway 29 and the wind at the time was from 060 at 17 gusting 24 knots. The density altitude of the airport was 5,300 feet.
The pilot of a Cessna C-175 flew into an airport with a field elevation of 5,130 feet MSL and a temperature in the 90 degree F range, producing a density altitude of 8,185 feet. The pilot then fueled the aircraft with auto gas and loaded three passengers for a sightseeing flight. He departed south even though the wind had shifted during the time he was on the ground to a northwest wind at 12 to 18 knots. The aircraft take-off roll was very long and when at about 100 feet AGL, the aircraft began to wobble, the right wing dropped and the aircraft descended into the ground.
The pilot of a Zenith CH-300 had recently modified his aircraft with the installation of a 25 imperial gallon auxiliary fuel tank. The tank was filled with fuel and the aircraft departed.Immediately after take-off the aircraft entered a left turn, hit some trees and crashed, fatally injuring the pilot.
The auxiliary fuel tank was installed behind the pilot and passenger seats and no weight and balance calculations could be found. The battery was located in the tail section and was unsecured.The reason the aircraft departed controlled flight was not determined, but it was speculated that the installation of the auxiliary tank and the rearward movement of the battery produced an aft C of G situation that may have resulted in control difficulties.
Two of the pilots above combined three things to produce their disasters — overweight aircraft, highdensity altitude and tail winds. The attitude that leads one to ignore one rule tends to lead one to ignore all of the rules. These two pilots may have had that attitude or they simply may have become complacent.
Taking off with a strong or gusty tail wind is foolish at any weight or any altitude.We all tend to get a little complacent, however, about being a little over our allowable gross weight and about the airport density altitude. The C-172 POH indicates that an increase of about 300 pounds will increase the take-off roll by about 30%.The KOCH Chart indicates that even at 3,000 feet on a hot day the take-off roll will increase by as much as 100% over standard conditions at sea level.
If we try to pull the aircraft airborne before it is ready to fly, the nose high attitude may get the aircraft airborne in ground effect, but we may not be able to climb out of it.
The overly long take-off rolls should have been a clue that something was wrong.
An aft C of G on departure can be, and probably was, fatal. The CH 300 likely stalled immediately after take-off.An aft C of G will allow the nose to lift off earlier on the take-off roll. The aircraft will become airborne earlier in ground effect at a speed that may be below the stall speed. If the aircraft is allowed to climb out of ground effect, it may stall. If the aircraft is held in ground effect, the high induced drag from the nose high attitude and high angle of attack may not allow the aircraft to accelerate to a safe airspeed in time to clear an obstacle off the end of the runway.
If we do get the aircraft airborne, an aft C of G in cruise flight makes the aircraft less stable along the longitudinal axis. An ex treme aft C of G makes the aircraft unstable and difficult to control. An aft C of G condition requires down elevator control to raise the tail to maintain level flight. This reduces the down load on the horizontal stabilizer and acts like a reduction in weight, allowing for a slightly faster airspeed for a given power setting and a lower stall speed.
As the aircraft slows down, there is less airflow over the horizontal stabilizer requiring more down elevator to maintain level flight, resulting in increased instability and more control difficulty.This could be a problem on final approach and landing. Fuel burn during flight in many aircraft results in the C of G moving aft. This will further increase the instability and control difficulty.
An aircraft with a forward C of G will require more aft control force to raise the nose for take-off. The result is usually a longer take-off distance. If the C of G is too far forward, the aircraft may not become airborne at all. Once airborne, the aircraft is more stable along the longitudinal axis, however more up elevator is required to maintain level flight. The farther forward the C of G the more up elevator required. This produces a down load on the tail of the aircraft that acts just like weight. The aircraft will fly more slowly at a given power setting and it will stall at a higher stall speed.
At slow speeds, there may not be enough up elevator available to keep the nose from falling forward, even at speeds well above the stall speed. This could result in a dangerous situation on final approach and landing.
Power will help keep the nose up, and conversely, any power reduction could result in a rapid pitch down of the nose of the aircraft.
Weight and balance are important. If we get complacent about them, we may bend some metal, or worse.
Dale Nielsen is an ex-Armed Forces pilot and aerial photography pilot. He lives in Abbotsford, B.C., and currently flies medevacs from Victoria in a Lear 25. Nielsen is also the author of seven flight training manuals published by Canuck West Holdings.