CASA opinion: Aircraft must be grounded in temps over 40 degrees
Perusing the old Digests kindly provided above in the forum, I came across this article in issue #33, which I reproduce in its entirety. Note density altitude is the focus, temp even goes to 45°C on the chart, and absolutely no mention of temp being a limit, just DA.
LIGHT AIRCRAFT TAKE-OFF PERFORMANCE
Temperature and Altitude Effects
A review of take-off accidents involving light aircraft has shown that an appreciable number of them can be attributed wholly or in part to a failure to allow for the effects of reduced air density arising from high temperature, high altitude, or, more particularly, from a combination of both.
Two separate effects must be considered;
(a) The effect of reduced air density on take-off
distance;
(b) the effect of reduced air density on climb
performance.
Both of these aspects will be examined in turn.
The Effect of Reduced Air Density on Take-off Distance
The normal takeoff consists of a full throttle run along the ground, a lift off at the take-off safety speed, and a climb away at this speed until a height of 50 feet is reached.
The take-off safety speed is defined as 1.2 Vs, where Vs is the power off stalling speed.
The indicated stalling speed of an aircraft depends, principally, on the aircraft’s weight, power setting and flap position. Changes in air density do not change the indicated air speed at the stall. Every pilot is aware, however, that under conditions of reduced air density the true air speed is greater than the indicated air speed; thus, in a take-off under high temperature conditions, the prescribed higher true air speed and the distance required to reach this speed will be greater. Alternatively, for a given take-off distance the gross weight of the aircraft and hence the take—off safety speed will have to be reduced in order to provide for a safe operation within the available distance.
Another major effect to be considered is the reduction of engine power output arising from reduced air density. In most light aircraft, take-off power is the full-throttle setting of its unsupercharged, or normally aspirated, engine. Changes in air density produce changes in the full throttle power of such engines. Any reduced air density means less air available for combustion and a fall-off in take-off power. The reduction in power is approximately proportional to the reduction in air density.* This reduction in available power means that less thrust will be available for accelerating or climbing the aircraft. lt can be seen, therefore, that reduced air density will not only demand longer take-off runs to allow the aircraft to accelerate to the higher true airspeeds but it also imposes the penalty of reducing the power available to achieve this acceleration. The take-off distances required are therefore greatly increased even for small reductions in air density.
The information provided in handbooks by the manufacturers of light aircraft is usually insufficient to take account of all the major variables and the Department of Civil Aviation has undertaken the production of the PL Charts (Performance Charts for Light Aircraft) to assist pilots in their calculations. For most aircraft types, the manufacturers data has been checked by flight testing in Australia and the chart data is based on these test results.
The chart indicates the maximum permissible gross weight for take-off after aerodrome pressure height, outside air temperature, take-off distance available and wind velocity are taken into account. Fifty per cent of the reported head wind component and 150 per cent of the reported tail wind component have been used in the construction of the chart and the take-off distance has been increased by a factor of 1.15 as is shown in the notes on the chart. The following example illustrated in the chart will show how the chart is used.
* In the case of a supercharged engine this effect is overcome, within limits, by compressing the air and thus restoring the air supply.
Airfield pressure height which may be read from your altimeter after setting 1,013.2 mb. = 920 feet
Outside air temperature measured in the shade : 113°F or 45°C
Take-off distance available = 1,550 feet
Wind velocity component = Nil
(1) Effect of Air Density Change
Enter the chart at °“START HERE" and find the intersection of the airfield pressure height (APH) and the outside air temperature (OAT). The point of intersection indicates the density height at which the next segment of the chart to the right should be entered. This density height is determined by the relationship of the APH/OAT intersection with the horizontal lines drawn through the upper three segments of the chart. The bottom line has a zero or standard sea level value as determined by the intersection of the zero airfield pressure and the standard 15°C temperature. Each successive line drawn is a 1,000 feet increment in density height. Thus it will be seen that the density height in this example is 4,500 foot which means that the density of the air under the conditions stated in the examples is the same as would exist at a height of 4,500 feet under conditions of standard atmosphere, At this point it is of interest to note the effect of temperature on density height. If the OAT had been 13‘”C the density height would have been the same as the airfield pressure height, i.e., 920 feet and, with an even lower temperature of 5°C (41°F) the equivalent of sea level standard conditions would prevail. It will become apparent from this why a light aircraft exhibits a lively performance on a frosty morning. Now move on to further corrections in the example.
(2) Effect of Take-off Distance Available
Move to the right on the chart until you intercept the line representing the take-off distance available and then move vertically downwards to the next correction.
It may be seen that, in the particular conditions of this take-off, no reduction of the maximum permissible gross weight would have been necessary had the available length of run been equal to or greater than 2,400 feet. Since the available length is only 1,550 feet, however, it is immediately apparent that the gross weight for take-off will need to be reduced.
(3) Effect of Wind
Continue to move vertically downwards to intercept the ambient wind velocity Line and then move horizontally to the left and read from the scale the maximum take-off weight permitted under these circumstances, i.e., 1.570 lb.
(4) Take-off Safety Speed
Since the stalling speed varies directly with weight, the take-off safety speed will also vary directly with the take-off weight and for this case it may be read directly off the right hand side of the diagram as 43 Kts. I.A.S.
We are now in a position to see that under the conditions prescribed, the combined effects of limited take-off distance available and the reduced air density has demanded a reduction in the maximum permissible take-otf weight from 1,825 lb. to 1,570 lb.
The Effect of Reduced Density on Climb Performance
The Australian performance standards require that all light aircraft have a minimum gradient of climb after take-off of six per cent, This can be expressed as 6 feet of climb for every 100 feet of horizontal travel along the flight path, or 365 feet per nautical mile which is equivalent to a rate of climb of 365 feet per minute if the aircraft’s climbing speed is 60 knots (T.A.S.).
The climb gradient is greatly affected by even a small reduction of engine power because the power available to climb the aircraft is only the power in excess of that required for straight and level flight at the climbing speed.
We have already pointed out that any reduction in air density produces a proportionate reduction in engine power. Reference to atmosphere tables will show that air density falls about 3 per cent per 1,000 feet between sea level and 3.000 feet, reducing to 2 per cent per 1,000 feet at 16,000 feet. Thus it the aircraft is taking off in conditions of pressure and temperature which are equivalent to a height of 4,500 feet under standard conditions (i.e., a density height of 4,500 feet) the engine output under full throttle at constant r.p.m. will fall about 13 per cent. This amounts to a considerable reduction in the power available for the climb and the gradient of climb is correspondingly reduced. If the density is reduced to a point where the minimum climb gradient would not be achieved, the take-off gross weight must be reduced in order to restore the gradient and thus ensure a safe climb out over obstacles.
To show how this adjustment is calculated we must now refer to the Climb Weight Limit diagram in the PL Chart.
Climb Weight Limit
Enter the chart at the airfield pressure height and move vertically until the line intercepts the outside air temperature. Then move horizontally to the left to the point of intersection with the sloping reference line and then vertically downwards to the gross weight scale where it can be seen that the climb weight limit is 1,775 lb.
Points to be Especially Noted
The maximum permissible weight derived from our previous calculations based on runway length available was 1,570 lb., Whilst the weight limitation based on the climb requirements is 1,775 lb. The lesser of these two is the maximum permissible take-off gross weight, i.e., 1,570 lb.
If the aircraft's gross weight is held constant the effect of temperature on the length required for take-off at a particular aerodrome may be seen from the chart. Referring to our example again, you will remember that 1,550 feet was the minimum length required to lift 1,570 lb. when the temperature was 45°C (113"F). Drop the temperature to 13°C (55°F), which is standard for a pressure height of 920 feet, and for the same weight the take-oil length required is reduced to 1,170 feet. Check this on the chart at the point where a density height of 920 feet intercepts the vertical line of our example in the ‘distance available" segment of the chart.
Whenever a take-off in the type of aircraft to which the sample chart applies is to be carried out with a density height exceeding 3,800 feet, Some reduction of take-off (i.e., 1,825 lb.) must be made irrespective of the length of run available. This arises from the climb weight limitations of the aircraft.
Example:
Now try this example yourself using a ruler and sharp pencil.
Airfield Pressure Height ....,. . ,.... 1.500 feet
Outside Air Temperature ..,... . ..... 25°C
Take—off distance available . .... 1,900 feet
Head Wind Component 5 m.p.h.
It you have mastered the system you will agree that the take—off gross weight is 1,785 lb. and the take-off safety speed is 47 Imots.
LIGHT AIRCRAFT TAKE-OFF PERFORMANCE
Temperature and Altitude Effects
A review of take-off accidents involving light aircraft has shown that an appreciable number of them can be attributed wholly or in part to a failure to allow for the effects of reduced air density arising from high temperature, high altitude, or, more particularly, from a combination of both.
Two separate effects must be considered;
(a) The effect of reduced air density on take-off
distance;
(b) the effect of reduced air density on climb
performance.
Both of these aspects will be examined in turn.
The Effect of Reduced Air Density on Take-off Distance
The normal takeoff consists of a full throttle run along the ground, a lift off at the take-off safety speed, and a climb away at this speed until a height of 50 feet is reached.
The take-off safety speed is defined as 1.2 Vs, where Vs is the power off stalling speed.
The indicated stalling speed of an aircraft depends, principally, on the aircraft’s weight, power setting and flap position. Changes in air density do not change the indicated air speed at the stall. Every pilot is aware, however, that under conditions of reduced air density the true air speed is greater than the indicated air speed; thus, in a take-off under high temperature conditions, the prescribed higher true air speed and the distance required to reach this speed will be greater. Alternatively, for a given take-off distance the gross weight of the aircraft and hence the take—off safety speed will have to be reduced in order to provide for a safe operation within the available distance.
Another major effect to be considered is the reduction of engine power output arising from reduced air density. In most light aircraft, take-off power is the full-throttle setting of its unsupercharged, or normally aspirated, engine. Changes in air density produce changes in the full throttle power of such engines. Any reduced air density means less air available for combustion and a fall-off in take-off power. The reduction in power is approximately proportional to the reduction in air density.* This reduction in available power means that less thrust will be available for accelerating or climbing the aircraft. lt can be seen, therefore, that reduced air density will not only demand longer take-off runs to allow the aircraft to accelerate to the higher true airspeeds but it also imposes the penalty of reducing the power available to achieve this acceleration. The take-off distances required are therefore greatly increased even for small reductions in air density.
The information provided in handbooks by the manufacturers of light aircraft is usually insufficient to take account of all the major variables and the Department of Civil Aviation has undertaken the production of the PL Charts (Performance Charts for Light Aircraft) to assist pilots in their calculations. For most aircraft types, the manufacturers data has been checked by flight testing in Australia and the chart data is based on these test results.
The chart indicates the maximum permissible gross weight for take-off after aerodrome pressure height, outside air temperature, take-off distance available and wind velocity are taken into account. Fifty per cent of the reported head wind component and 150 per cent of the reported tail wind component have been used in the construction of the chart and the take-off distance has been increased by a factor of 1.15 as is shown in the notes on the chart. The following example illustrated in the chart will show how the chart is used.
* In the case of a supercharged engine this effect is overcome, within limits, by compressing the air and thus restoring the air supply.
Airfield pressure height which may be read from your altimeter after setting 1,013.2 mb. = 920 feet
Outside air temperature measured in the shade : 113°F or 45°C
Take-off distance available = 1,550 feet
Wind velocity component = Nil
(1) Effect of Air Density Change
Enter the chart at °“START HERE" and find the intersection of the airfield pressure height (APH) and the outside air temperature (OAT). The point of intersection indicates the density height at which the next segment of the chart to the right should be entered. This density height is determined by the relationship of the APH/OAT intersection with the horizontal lines drawn through the upper three segments of the chart. The bottom line has a zero or standard sea level value as determined by the intersection of the zero airfield pressure and the standard 15°C temperature. Each successive line drawn is a 1,000 feet increment in density height. Thus it will be seen that the density height in this example is 4,500 foot which means that the density of the air under the conditions stated in the examples is the same as would exist at a height of 4,500 feet under conditions of standard atmosphere, At this point it is of interest to note the effect of temperature on density height. If the OAT had been 13‘”C the density height would have been the same as the airfield pressure height, i.e., 920 feet and, with an even lower temperature of 5°C (41°F) the equivalent of sea level standard conditions would prevail. It will become apparent from this why a light aircraft exhibits a lively performance on a frosty morning. Now move on to further corrections in the example.
(2) Effect of Take-off Distance Available
Move to the right on the chart until you intercept the line representing the take-off distance available and then move vertically downwards to the next correction.
It may be seen that, in the particular conditions of this take-off, no reduction of the maximum permissible gross weight would have been necessary had the available length of run been equal to or greater than 2,400 feet. Since the available length is only 1,550 feet, however, it is immediately apparent that the gross weight for take-off will need to be reduced.
(3) Effect of Wind
Continue to move vertically downwards to intercept the ambient wind velocity Line and then move horizontally to the left and read from the scale the maximum take-off weight permitted under these circumstances, i.e., 1.570 lb.
(4) Take-off Safety Speed
Since the stalling speed varies directly with weight, the take-off safety speed will also vary directly with the take-off weight and for this case it may be read directly off the right hand side of the diagram as 43 Kts. I.A.S.
We are now in a position to see that under the conditions prescribed, the combined effects of limited take-off distance available and the reduced air density has demanded a reduction in the maximum permissible take-otf weight from 1,825 lb. to 1,570 lb.
The Effect of Reduced Density on Climb Performance
The Australian performance standards require that all light aircraft have a minimum gradient of climb after take-off of six per cent, This can be expressed as 6 feet of climb for every 100 feet of horizontal travel along the flight path, or 365 feet per nautical mile which is equivalent to a rate of climb of 365 feet per minute if the aircraft’s climbing speed is 60 knots (T.A.S.).
The climb gradient is greatly affected by even a small reduction of engine power because the power available to climb the aircraft is only the power in excess of that required for straight and level flight at the climbing speed.
We have already pointed out that any reduction in air density produces a proportionate reduction in engine power. Reference to atmosphere tables will show that air density falls about 3 per cent per 1,000 feet between sea level and 3.000 feet, reducing to 2 per cent per 1,000 feet at 16,000 feet. Thus it the aircraft is taking off in conditions of pressure and temperature which are equivalent to a height of 4,500 feet under standard conditions (i.e., a density height of 4,500 feet) the engine output under full throttle at constant r.p.m. will fall about 13 per cent. This amounts to a considerable reduction in the power available for the climb and the gradient of climb is correspondingly reduced. If the density is reduced to a point where the minimum climb gradient would not be achieved, the take-off gross weight must be reduced in order to restore the gradient and thus ensure a safe climb out over obstacles.
To show how this adjustment is calculated we must now refer to the Climb Weight Limit diagram in the PL Chart.
Climb Weight Limit
Enter the chart at the airfield pressure height and move vertically until the line intercepts the outside air temperature. Then move horizontally to the left to the point of intersection with the sloping reference line and then vertically downwards to the gross weight scale where it can be seen that the climb weight limit is 1,775 lb.
Points to be Especially Noted
The maximum permissible weight derived from our previous calculations based on runway length available was 1,570 lb., Whilst the weight limitation based on the climb requirements is 1,775 lb. The lesser of these two is the maximum permissible take-off gross weight, i.e., 1,570 lb.
If the aircraft's gross weight is held constant the effect of temperature on the length required for take-off at a particular aerodrome may be seen from the chart. Referring to our example again, you will remember that 1,550 feet was the minimum length required to lift 1,570 lb. when the temperature was 45°C (113"F). Drop the temperature to 13°C (55°F), which is standard for a pressure height of 920 feet, and for the same weight the take-oil length required is reduced to 1,170 feet. Check this on the chart at the point where a density height of 920 feet intercepts the vertical line of our example in the ‘distance available" segment of the chart.
Whenever a take-off in the type of aircraft to which the sample chart applies is to be carried out with a density height exceeding 3,800 feet, Some reduction of take-off (i.e., 1,825 lb.) must be made irrespective of the length of run available. This arises from the climb weight limitations of the aircraft.
Example:
Now try this example yourself using a ruler and sharp pencil.
Airfield Pressure Height ....,. . ,.... 1.500 feet
Outside Air Temperature ..,... . ..... 25°C
Take—off distance available . .... 1,900 feet
Head Wind Component 5 m.p.h.
It you have mastered the system you will agree that the take—off gross weight is 1,785 lb. and the take-off safety speed is 47 Imots.
It seems you can delude yourself into believing anything if the circumstances dictate.
Last edited by KRUSTY 34; 23rd Oct 2017 at 04:48.
The SAAB AOM does state a limitation of 47 degrees C.
By whom?
With what equipment? Mercury thermometers, the common rotary dial gauges & digital gauges all rear differently with different reaction times or lag
Where? Engine compartment, ambient? ambient in the shade, ambient at aerodrome reference?
The limitation probably relates to hot fuel handling of the engine - which will have more to do with heat soak than ambient conditions.
The role of a pilot is to make judgement decisions, this is one of them .
Megan, if you could pdf that letter and upload would do wonders for those not game enough to take it up to uneducated FOIs on the principles of flight. Density could be an argument....maybe too much of it between the ears
Or, grey matter lacking oxygen starving mental performance
Or, hot heads lack performance when it counts....bloody lawyers! Obfuscators Lacking Competency!
Or, grey matter lacking oxygen starving mental performance Or, hot heads lack performance when it counts....bloody lawyers! Obfuscators Lacking Competency!
Getting into the hot weather down here so time to consider this again. (I only deal with small airplanes.)
CAO 20.7.4 Aeroplane weight and performance limitations — aeroplanes not above 5 700 kg — private, aerial work (excluding agricultural) and charter operations states "Approved declared conditions may be used instead of actual pressure height and temperature" - the declared conditions are from CAO 20.7.0 as declared density altitude.
A couple of years ago, a note in the Exposure Draft of Part 91 (MOS for 91.1035 Aircraft Performance): “It is the intention that CAOs 20.7.4. …. will be subject of a project to review them and provide guidance material in the form of an AC in the future. Much of the content of the CAOs contain either certification standards or outdated information. CASA expects pilots to operate in accordance with the aircraft flight manual (AFM). All performance information in the AFM is produced and complies with the aeroplane certification standards.”
I've heard nought of that project since.
If one has a FAR 23 airplane not more than 6,000 lb maximum weight there are no performance weight limitations in the AFM so just work your way through CAO 20.7.4 and try to deal with the outdated information and Australian-specific certification standards within it.
A couple of years ago, a note in the Exposure Draft of Part 91 (MOS for 91.1035 Aircraft Performance): “It is the intention that CAOs 20.7.4. …. will be subject of a project to review them and provide guidance material in the form of an AC in the future. Much of the content of the CAOs contain either certification standards or outdated information. CASA expects pilots to operate in accordance with the aircraft flight manual (AFM). All performance information in the AFM is produced and complies with the aeroplane certification standards.”
I've heard nought of that project since.
If one has a FAR 23 airplane not more than 6,000 lb maximum weight there are no performance weight limitations in the AFM so just work your way through CAO 20.7.4 and try to deal with the outdated information and Australian-specific certification standards within it.
