BayAreaLondoner
16th May 2001, 02:43
A while ago in the Tech Log forum, a thread was started called Rules of Thumb.
I happened to come across a whole list of rules of thumb in the US AOPA's Pilot's Handbook. They are pasted below in case others are interested:
The ability of the atmosphere to hold moisture doubles with each 11°-Celsius temperature rise.
When the wind aloft is more southerly and stronger than forecast, it means that the weather may become worse than forecast — especially if the temperature aloft is warmer than forecast. Higher temperature means the atmosphere can hold more moisture. More southerly and stronger winds mean there is a stronger than forecast low or front or trough to the west, heading your way (Northern Hemisphere only).
The standard temperature (ISA) is 15°C at sea level, and it decreases 2° for each 1,000 feet. To calculate standard at altitude, multiply the altitude (in thousands) by two and subtract that number from 15. At Flight Level 210, 15 - 42 = -27, which is the standard temperature at that altitude.
If the winds aloft velocity is 10 percent of the true airspeed, there will be 4° of drift if it is 45° off the nose or tail and 6° if it is a 90° crosswind.
Seventy knots is 118 feet per second, and 60 is 101 fps. So if the approach speed should have been 60 knots and is 70, and if it takes five seconds to dissipate the extra speed, the airplane will have traveled about 550 feet in the float. No firm rule of thumb, but 10 knots extra on the approach speed usually uses about 500 extra feet of runway.
The air is conditionally unstable if the temperature drops more than 2° per 1,000 feet on ascent.
When the surface wind shifts to the north or northeast after passage of a cold front, that front may well be back as a warm front in a day or so.
To descend 500 feet per minute to the destination, start the descent 5 miles out for each 1,000 feet to be lost if the groundspeed is 150 knots. For each 30 knots in either direction, add or subtract 100 fpm. At 180 knots, you'd need 600 fpm; at 450 knots, 1,500 fpm.
A VOR course deviation indicator reflects 10° off course when full scale in either direction. One degree equals 1 mile when the aircraft is 60 miles from the station, so if you are 60 miles out with a full scale, you are 10 miles off course. If 30 miles out and a half scale (5°), you would be 2.5 miles off course.
Performance speeds — such as maneuvering, approach, and climb speeds — are often given in the POH only for operations at gross weight. To calculate speeds for lighter weights, decrease the speed by half the percentage of the weight decrease. For example, flying a 3,000-pound-gross airplane at 2,400 pounds, a 20-percent reduction in weight, reduce the applicable speeds by 10 percent to hold the margins the same as at gross.
How long to get there? If the groundspeed is 150 knots, multiply the distance by four and drop the last zero: 20 miles × 4 = 80. It will take eight minutes at 150. For 100, multiply by six for 120, minus the zero for 12 minutes. For 120, just divide by two; 180 means dividing by three.
Maneuvering speed is stall speed multiplied by the square root of the limit load factor. Normal category limit is 3.8 Gs, the square root of which is 1.95. If, for example, the flaps-up stalling speed is 70, the maneuvering speed would be 70 × 1.95 = 136.5.
Specific fuel consumption is the measure of how many pounds of fuel an engine burns per hour to make 1 horsepower. Properly leaned, the most efficient engines are about 0.4, with a good average being 0.42 or 0.43. Turbocharged engines wander up to 0.45 or 0.475. Say you have a 200-hp engine that is fairly efficient at 0.42, and you are flying along burning 12 gallons an hour and want to calculate the amount of horsepower being used. Twelve gallons is 72 pph ÷ 0.42 = 171.4 hp, or almost 86-percent power. Going the other way, 75 percent is 150 hp × 0.42 for 63 pph.
To make an approximate calculation of the bases of fair-weather cumulus, divide the temperature/dew-point spread by four: 84/60 would mean the cloud bases would be somewhere around 6,000 feet.
Winds aloft velocity almost always increases in a frontal zone. This is seldom reflected in forecasts, and depending on the strength of the front, you'll likely see an increase over the forecast value from about 200 miles ahead of the front to 200 miles behind the front.
In a normally aspirated airplane, add about 3 pounds per cylinder to total fuel burn for the extra amount required to take off and climb to cruise. For start and taxi fuel, the time has to be known. A ballpark figure for idling fuel flow is from 15 to 20 pph, depending on the engine.
[This message has been edited by BayAreaLondoner (edited 15 May 2001).]
[This message has been edited by BayAreaLondoner (edited 15 May 2001).]
I happened to come across a whole list of rules of thumb in the US AOPA's Pilot's Handbook. They are pasted below in case others are interested:
The ability of the atmosphere to hold moisture doubles with each 11°-Celsius temperature rise.
When the wind aloft is more southerly and stronger than forecast, it means that the weather may become worse than forecast — especially if the temperature aloft is warmer than forecast. Higher temperature means the atmosphere can hold more moisture. More southerly and stronger winds mean there is a stronger than forecast low or front or trough to the west, heading your way (Northern Hemisphere only).
The standard temperature (ISA) is 15°C at sea level, and it decreases 2° for each 1,000 feet. To calculate standard at altitude, multiply the altitude (in thousands) by two and subtract that number from 15. At Flight Level 210, 15 - 42 = -27, which is the standard temperature at that altitude.
If the winds aloft velocity is 10 percent of the true airspeed, there will be 4° of drift if it is 45° off the nose or tail and 6° if it is a 90° crosswind.
Seventy knots is 118 feet per second, and 60 is 101 fps. So if the approach speed should have been 60 knots and is 70, and if it takes five seconds to dissipate the extra speed, the airplane will have traveled about 550 feet in the float. No firm rule of thumb, but 10 knots extra on the approach speed usually uses about 500 extra feet of runway.
The air is conditionally unstable if the temperature drops more than 2° per 1,000 feet on ascent.
When the surface wind shifts to the north or northeast after passage of a cold front, that front may well be back as a warm front in a day or so.
To descend 500 feet per minute to the destination, start the descent 5 miles out for each 1,000 feet to be lost if the groundspeed is 150 knots. For each 30 knots in either direction, add or subtract 100 fpm. At 180 knots, you'd need 600 fpm; at 450 knots, 1,500 fpm.
A VOR course deviation indicator reflects 10° off course when full scale in either direction. One degree equals 1 mile when the aircraft is 60 miles from the station, so if you are 60 miles out with a full scale, you are 10 miles off course. If 30 miles out and a half scale (5°), you would be 2.5 miles off course.
Performance speeds — such as maneuvering, approach, and climb speeds — are often given in the POH only for operations at gross weight. To calculate speeds for lighter weights, decrease the speed by half the percentage of the weight decrease. For example, flying a 3,000-pound-gross airplane at 2,400 pounds, a 20-percent reduction in weight, reduce the applicable speeds by 10 percent to hold the margins the same as at gross.
How long to get there? If the groundspeed is 150 knots, multiply the distance by four and drop the last zero: 20 miles × 4 = 80. It will take eight minutes at 150. For 100, multiply by six for 120, minus the zero for 12 minutes. For 120, just divide by two; 180 means dividing by three.
Maneuvering speed is stall speed multiplied by the square root of the limit load factor. Normal category limit is 3.8 Gs, the square root of which is 1.95. If, for example, the flaps-up stalling speed is 70, the maneuvering speed would be 70 × 1.95 = 136.5.
Specific fuel consumption is the measure of how many pounds of fuel an engine burns per hour to make 1 horsepower. Properly leaned, the most efficient engines are about 0.4, with a good average being 0.42 or 0.43. Turbocharged engines wander up to 0.45 or 0.475. Say you have a 200-hp engine that is fairly efficient at 0.42, and you are flying along burning 12 gallons an hour and want to calculate the amount of horsepower being used. Twelve gallons is 72 pph ÷ 0.42 = 171.4 hp, or almost 86-percent power. Going the other way, 75 percent is 150 hp × 0.42 for 63 pph.
To make an approximate calculation of the bases of fair-weather cumulus, divide the temperature/dew-point spread by four: 84/60 would mean the cloud bases would be somewhere around 6,000 feet.
Winds aloft velocity almost always increases in a frontal zone. This is seldom reflected in forecasts, and depending on the strength of the front, you'll likely see an increase over the forecast value from about 200 miles ahead of the front to 200 miles behind the front.
In a normally aspirated airplane, add about 3 pounds per cylinder to total fuel burn for the extra amount required to take off and climb to cruise. For start and taxi fuel, the time has to be known. A ballpark figure for idling fuel flow is from 15 to 20 pph, depending on the engine.
[This message has been edited by BayAreaLondoner (edited 15 May 2001).]
[This message has been edited by BayAreaLondoner (edited 15 May 2001).]