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Gremline Flight Safety Report: Effects of Air Density on Aircraft Performance / Drug Effects

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Text Box: drug effects / air density effects

the gremline digest —  drug effects / effects of air density

In Brief … How Long Do Drug Effects Last?

The AAIB Report into a fatal accident where the pilot apparently lost control of his aircraft in marginal weather and impacted the ground at a steep attitude at high speed contains some interesting comments from the specialist aviation pathologist who carried out the post-mortem examination of the pilot. No medical cause for the accident was identified. Samples were sent to a laboratory for toxicological analysis and the laboratory report stated that:

 

‘Toxicology revealed the presence of tetrahydrocannabinoic acid (THC-COOH) in the pilot’s blood. This is an inactive metabolite of tetrahydrocannabol (THC) which is the main active constituent of cannabis. THC concentrations generally fall below 5 mg/ml less than three hours after smoking cannabis, and are generally below the limits of quantification within eight to twelve hours. In contrast, THC-COOH is excreted from the body over a period of days to weeks. Consequently, the results in this case indicate that cannabis had been consumed at some stage prior to the flight, but the absence of THC indicates that this would not have been within the few hours immediately preceding the flight.
While THC was not detected, this does not necessarily mean that the pilot would have been unaffected by cannabis. Effects have been demonstrated on attention, psychomotor tasks and short term memory during the 12 to 24 hours following cannabis use, and an adverse affect on performance of complex cognitive tasks has been demonstrated 24 hours after smoking cannabis. The drug can have a detrimental effect on psychomotor control long after it has ceased to exert any of the euphoric effects for which it is taken and long after the user perceives that there is any effect.’

This information is taken from AAIB Report EW/C2008/10/01 which source is gratefully acknowledged.

 

 

The Effects of Air Density on Aircraft Performance

Recreational pilots may not always consider the effects of air density on the performance of their aircraft while calculating take-off performance. If we go back to the basic definition of the amount of lift that is generated by a wing then we get to the formula “lift equals the coefficient of lift for our aerofoil section times the dynamic pressure times the area of our wing”.

 

This is the familiar formula L=Cl½ρV²S. Note that one symbol in this formula is the Greek letter ρ. This symbol “rho” refers to the DENSITY of the air. Without going too deeply, it is obvious that a decrease in “rho”, the air density, will result in a decrease in the total lift generated by our wing at a given airspeed. An increase in the air temperature will result in a decrease in the air density. The air density is also affected by the humidity of the air in that the density decreases with increased humidity.
      What has all this got to do with performance? There are two things that need to be remembered about air density. It decreases on hot days and on humid days. This affects both the amount of lift that will be generated by your wing and also the amount of power that will be generated by the engine of your aircraft. I’m sure that most pilots have noticed that the engine produces more power on a cold and dry day than it does on a hot, humid day. Well, these conditions also affect the amount of lift generated by the wing at a given speed.
The practical results are that you will require a longer take-off run when the air density is reduced than you need when the atmosphere is close to that of a Standard Atmosphere – which includes a temperature of +15ºC.

 


A recent accident highlights the effect a decrease in air density has on take-off performance.
An Auster Aiglet Trainer was taking off from Bicester on a September afternoon with a temperature of +25ºC and a dew point of 13ºC. There were two people on board.
The pilot began the takeoff run some 150 to 200 m from the start of the 1000 m long grass runway. Witnesses thought the aircraft was slow to accelerate but it then “lurched upwards” as if the pilot was trying to “haul the aircraft into the air”. The aircraft began to climb but only gently and once again it was seen to “lurch” upwards as it approached a line of trees. As it passed low over the trees, the left wing and the nose dropped and the aircraft descended into the trees and came to rest in the corner of a small industrial site approximately 380 m beyond the and of the runway. The passenger was helped from the wreckage soon after the impact but the pilot had to be cut from the wreckage before being flown to hospital.
      An experienced Auster pilot saw the accident from a point close to the start of the take-off run. He recalled that the aircraft used a lot of runway and was in a slightly nose-down attitude rather than level or slightly nose-up as he would have expected. He confirmed that the aircraft lifted off about ¾ of the way along the runway. As it left the ground the nose “pitched up noticeably” and he saw some “pilot inducted oscillations” in pitch. The aircraft settled into quite a nose-high attitude but was only climbing slowly and began to turn right gently. The witness then saw the “left wing drop and the nose yaw left”. The aircraft was only a few feet above the trees when it rolled approximately 60º left and the nose pitched down. This witness was one of the first people on the scene. He reported that the elevator was in the full nose-up position rather than neutral, which would have been the norm for take-off. He also noticed that the flaps were set to the second position whereas they would usually be at the first position for take-off.
      The pilot survived and was able to remember some of the events leading up to the take-off. He decided to take off on Runway 36 although there was a very light southerly wind indicated by a windsock on the southern end of the airfield. He expected the surface wind to become a crosswind as he approached mid-point in his take-off. He considered that the remaining distance available of about 850 m would be sufficient. He said that, although he could not remember the actual trim position, full nose-up trim would have required more force than normal to raise the tail which might have prompted him to reject the take-off. He said he had never used two stages of flap for take off and wondered if he had lowered the flap in an attempt to clear the trees.

 

 

AAIB Analysis

The airfield is at an altitude of 267 ft amsl but in the conditions of the day its density altitude was approximately 1270 ft. The aircraft started its takeoff run approximately 150 to 200 m inset from the start of the runway although there was approximately 800 to 850 m still available. It is possible that there was a very slight tailwind during the early part of the takeoff run. The nose-down attitude of the aircraft would have resulted in a greater down force on the tyres than usual, which was likely to have reduced the acceleration. The higher density altitude would also have led to an acceleration that was less than usual. The combination of factors contributed to a longer ground run, and a lift-off point further along the runway, than would have otherwise been expected. Once airborne, the aircraft’s climb performance would probably have been reduced by the high density altitude and the aircraft might not have accelerated at its usual rate. There is no evidence that the aircraft hit the trees before the loss of control but its clearance from them was marginal. It is possible that the “lurch” upwards as the aircraft approached the trees represents an attempt by the pilot to clear the tree line, perhaps by lowering a stage of flap. The evidence suggests that the aircraft stalled with an accompanying wing drop at such a low height above the trees that recovery was impossible. It was not possible to positively determine the flap or trim settings during the takeoff ground roll.

      The facts relating to this accident are taken from AAIB Report EW/G2009/09/15 which source is gratefully acknowledged. The comments are those of the Gremline editor and do not represent the views of AAIB.

 

If you would like further information on density altitude and its effect on performance have a look at http://wahiduddin.net/calc/density_altitude.htm where you will find all you need to know plus a density altitude calculator. If your aircraft has a Rotax engine you will also find that Rotax advises that their carburettor main jet diameter should be adjusted according to air density, and includes a calculator to help you select the right size.

      Most navigation computers ( or ‘whizz wheels’) allow you to calculate density altitude for the conditions prevailing prior to takeoff. A ‘guesstimate’ may be achieved by the formula “Density altitude in feet = pressure altitude in feet + (120x(OAT-ISA temp) where the ISA temp=15º-(1.98ºC/1000 ft). In the condition pertaining at the time of this accident this ‘guesstimate’ formula would have produced an answer of about 1400 feet, which although not very accurate is enough to alert the pilot to the degradation in performance.

 

Be aware of density altitude on hot humid days.

 

 

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