Post by Tom Goodrick on Aug 24, 2008 17:08:01 GMT -5
There are several factors to consider in making proper descents: 1) getting down in time, 2) keeping airspeed within bounds, 3) getting to a position from which an approach can be made to the runway.
GETTING DOWN IN TIME
You want to get down, in general, about 10 nm from the airport. When you know your airplane well and can navigate well to a position for the approach, you can shorten this distance. Start with 10 nm. Then divide the average true airspeed by 60 (rough guess). This is how far forward you go in each minute. Next divide the altitude you must lose by your average descent rate. Since you will probably be descending on autopilot, you'll be selecting this descent rate and holding it constant. Multiply the number of minutes of descent by the amount you move forward each minute and add this to the 10 miles you want at the end of descent. This tells you how far out you must begin your descent. Another way to do this is best used in jets at altitudes above 30,000 ft. Take your altitude in thousands of feet and multiply by 3. Add 10 nm and this is the distance from the airport at which you should begin your descent. Of course you need to persuade ATC to agree with you. Descend at 2,000 fpm. ATC will often keep you high too long and then ask you to descend very quickly. Don't feel you have to try to do the impossible. Your job is to fly the plane safely.
KEEPING AIRSPEED WITHIN BOUNDS
A good rule of thumb is that indicated airspeed during descent should be about the same as it was during climb. At lower altitudes it should be about twice the clean stall speed. The power should be pulled back to about 30% for the descent. It is good to do this before actually starting the descent so the plane begins to slow down before it starts going downhill. Piston engines cannot be reduced to idle quickly when starting a descent. This would ruin the engine. Power reduction should be made in stages: 75% to 55% to 40% to 30%. A typical descent power for pistons is 2100 rpm and 15 inches MAP. You would normally bring the throttle to 20 inches MAP and the RPM to 2100. Then after a bit set 15 inches MAP.
GETTING TO AN APPROACH POSITION
An approach position is 5 to 8 nm downwind of the runway and 2000 to 3000 ft above the runway elevation if terrain permits. ATC will often try to get you 20 nm downwind of the runway. When you get there you will have to turn to the runway heading (020 degrees for runway 2, etc). Sit down with a drawing compass and a piece of paper to work out the problems of vectors to positions relative to various runways and how you can turn for alignment with the runway. Use a radious of turn of 0.5 nm for piston aircraft and 1 nm for jets. Draw the runway and then a line two minutes long in the down wind direction (3.3 nm for 100 knots). Make a light construction line perpendicular to the end of this two-minute line. Draw curves through 180 degrees of turn starting on the line toward the runway and moving back away from the runway centered on the construction line. You want to fly a path that is tangent to one of those circles. once on the circle you begin a 30 degree bank until aligned with the runway. (This works best in little or no wind.)
During descent, listen to ATIS to get the number of the runway in use. Envision a rough approximation of that approach geometry. If using ATC, ATC will get you in position for the approach.
FLYING THE DESCENT
You do have to watch the airspeed as the descent gets started. The indicated airspeed should always stay below that used in cruise. If there is turbulence, the airspeed should be held on the maneuvering speed, Va, which is twice the clean stall speed. You should not push forward much on the stick to start the descent. Merely reducing the power and setting the autopilot for the final altitude will be adequate. If hand flying just a slight nudge forward on the stick may be helpful. In jets it is common to use the spoilers at the beginning of the descent to help get the airspeed down. In some cases where ATC leaves you high too long, The spoilers can be kept out with the descent rate building to 3000 fpm or more until well into the descent.
Any extra speed carried during the descent can be bled off after levelling at the final altitude by keeping the power set as for descent. This should get you to the airspeed for lowering the gear. In some cases first flaps can be set before the gear is lowered. Then moderate power is required to maintain level flight until ready for the final descent to the runway.
As the descent is made, check the aircraft weight, given the remaining fuel. Most aircraft have a landing weight restriction that is lower than the max takeoff weight. Both in real life and in FS9, the landing gear will not handle the landing loads very well if you are heavier than the landing weight. In FS you can change the fuel weight if you are too heavy. Most of my panels have weight displayed continuously by the flight management computer.
MANUAL FLYING
Though the autopilot can do a decent job of flying final approach if you set it up right to intercept the ILS, you must still take over manual control to make the landing with most aircraft. To me, the idea of switching to manual control when low and slow a half-mile out is ridiculous and asking for trouble. I switch to manual control shortly after levelling off in the vicinity of the airport. You need some time to adjust to manual control while the aircraft has plenty of altitude and airspeed.
One key to good manual flying is to have the pitch trim set for the conditions so you do not have to use much force in the fore and aft direction on the stick or yoke to keep the plane flying level. If the plane has flown on autopilot for a few minutes after levelling off, the trim will already be set properly. You just have to make small adjustments during the rest of the flight. Whenever you want to change speed, the trim should be adjusted slightly. Get used to it. You'll be dropping the wheels. Trim for that. Extend the first amount of flaps. Trim for that. Add power. Trim for that. Get used to doing this so it becomes second nature. You should never make significant pitch corrections with the stick except for two occasions during a flight - to rotate for takeoff and to rotate for landing flare. (Of course that assumes you never screw up and get close to a stall.)
The throttle makes the plane go up or down. Add power and it climbs. Reduce power and it descends. You don't have to move the stick to make these things happen.
Next we will talk about making the final approach and landing.
GETTING DOWN IN TIME
You want to get down, in general, about 10 nm from the airport. When you know your airplane well and can navigate well to a position for the approach, you can shorten this distance. Start with 10 nm. Then divide the average true airspeed by 60 (rough guess). This is how far forward you go in each minute. Next divide the altitude you must lose by your average descent rate. Since you will probably be descending on autopilot, you'll be selecting this descent rate and holding it constant. Multiply the number of minutes of descent by the amount you move forward each minute and add this to the 10 miles you want at the end of descent. This tells you how far out you must begin your descent. Another way to do this is best used in jets at altitudes above 30,000 ft. Take your altitude in thousands of feet and multiply by 3. Add 10 nm and this is the distance from the airport at which you should begin your descent. Of course you need to persuade ATC to agree with you. Descend at 2,000 fpm. ATC will often keep you high too long and then ask you to descend very quickly. Don't feel you have to try to do the impossible. Your job is to fly the plane safely.
KEEPING AIRSPEED WITHIN BOUNDS
A good rule of thumb is that indicated airspeed during descent should be about the same as it was during climb. At lower altitudes it should be about twice the clean stall speed. The power should be pulled back to about 30% for the descent. It is good to do this before actually starting the descent so the plane begins to slow down before it starts going downhill. Piston engines cannot be reduced to idle quickly when starting a descent. This would ruin the engine. Power reduction should be made in stages: 75% to 55% to 40% to 30%. A typical descent power for pistons is 2100 rpm and 15 inches MAP. You would normally bring the throttle to 20 inches MAP and the RPM to 2100. Then after a bit set 15 inches MAP.
GETTING TO AN APPROACH POSITION
An approach position is 5 to 8 nm downwind of the runway and 2000 to 3000 ft above the runway elevation if terrain permits. ATC will often try to get you 20 nm downwind of the runway. When you get there you will have to turn to the runway heading (020 degrees for runway 2, etc). Sit down with a drawing compass and a piece of paper to work out the problems of vectors to positions relative to various runways and how you can turn for alignment with the runway. Use a radious of turn of 0.5 nm for piston aircraft and 1 nm for jets. Draw the runway and then a line two minutes long in the down wind direction (3.3 nm for 100 knots). Make a light construction line perpendicular to the end of this two-minute line. Draw curves through 180 degrees of turn starting on the line toward the runway and moving back away from the runway centered on the construction line. You want to fly a path that is tangent to one of those circles. once on the circle you begin a 30 degree bank until aligned with the runway. (This works best in little or no wind.)
During descent, listen to ATIS to get the number of the runway in use. Envision a rough approximation of that approach geometry. If using ATC, ATC will get you in position for the approach.
FLYING THE DESCENT
You do have to watch the airspeed as the descent gets started. The indicated airspeed should always stay below that used in cruise. If there is turbulence, the airspeed should be held on the maneuvering speed, Va, which is twice the clean stall speed. You should not push forward much on the stick to start the descent. Merely reducing the power and setting the autopilot for the final altitude will be adequate. If hand flying just a slight nudge forward on the stick may be helpful. In jets it is common to use the spoilers at the beginning of the descent to help get the airspeed down. In some cases where ATC leaves you high too long, The spoilers can be kept out with the descent rate building to 3000 fpm or more until well into the descent.
Any extra speed carried during the descent can be bled off after levelling at the final altitude by keeping the power set as for descent. This should get you to the airspeed for lowering the gear. In some cases first flaps can be set before the gear is lowered. Then moderate power is required to maintain level flight until ready for the final descent to the runway.
As the descent is made, check the aircraft weight, given the remaining fuel. Most aircraft have a landing weight restriction that is lower than the max takeoff weight. Both in real life and in FS9, the landing gear will not handle the landing loads very well if you are heavier than the landing weight. In FS you can change the fuel weight if you are too heavy. Most of my panels have weight displayed continuously by the flight management computer.
MANUAL FLYING
Though the autopilot can do a decent job of flying final approach if you set it up right to intercept the ILS, you must still take over manual control to make the landing with most aircraft. To me, the idea of switching to manual control when low and slow a half-mile out is ridiculous and asking for trouble. I switch to manual control shortly after levelling off in the vicinity of the airport. You need some time to adjust to manual control while the aircraft has plenty of altitude and airspeed.
One key to good manual flying is to have the pitch trim set for the conditions so you do not have to use much force in the fore and aft direction on the stick or yoke to keep the plane flying level. If the plane has flown on autopilot for a few minutes after levelling off, the trim will already be set properly. You just have to make small adjustments during the rest of the flight. Whenever you want to change speed, the trim should be adjusted slightly. Get used to it. You'll be dropping the wheels. Trim for that. Extend the first amount of flaps. Trim for that. Add power. Trim for that. Get used to doing this so it becomes second nature. You should never make significant pitch corrections with the stick except for two occasions during a flight - to rotate for takeoff and to rotate for landing flare. (Of course that assumes you never screw up and get close to a stall.)
The throttle makes the plane go up or down. Add power and it climbs. Reduce power and it descends. You don't have to move the stick to make these things happen.
Next we will talk about making the final approach and landing.