Post by Tom Goodrick on Oct 3, 2009 10:48:22 GMT -5
While flying turns manually while holding altitude is a nice challenge for a pilot, the most important job he has is to get the airplane up to cruise altitude and then back down to pattern altitude. We all know what to do from pattern altitude to the runway. But how many of us know what to do to make the various airplanes go up and down?
It might be interesting to discuss this topic. We can use various aircraft as examples. It can be challenging in any aircraft. The basic problem in climbing is that airplanes run out of power with altitude so that they must be flown carefully to maintain a safe airspeed with a reasonable climb rate. In some aircraft a "reasonable" climb rate is anything you can get with an airspeed 30% over stall. After takeoff in any airplane, I turn on the autopilot to make turns toward the departure path and to control climb. I always use the altitude controls (ALT mode) with either HDG mode or NAV mode for steering. In ALT mode your only control is the vertical speed setting. You can start climbing with a good vertical speed but you must adjust this with altitude to keep the airspeed from getting too low. Take the clean stall speed and add 30% to handle wind shear or turbulence. That is your minimum indicated airspeed at any altitude during the climb. in a simple non-tubocharged piston aircraft, you have the additional burden of adjusting the mixture control to maintain max power (or max fuel flow) during the climb. In all other aircraft the vertical speed is your only control.
Descent has some challenges for all aircraft. You have to decide, based on average ground speed and descent rate how far from the destination you will begin your descent. From flight levels above Fl200 the rule is x=3*H/1000. Divide your altitude by 1000 and multiply by 3. In a turboprop or pressurized piston, you'll be at least 60 nm from the destination when you start down. In a bizjet, you'll be 120 to 150 nm from the destination. in am unpressurized piston, you will descend at 500 fpm for ear comfort. So you have to figure 2 minutes per thousand feet. estimate distance based on your average ground speed in n miles per minute. (Divide knots by 60.) Your altitude over pattern altitude divided by 2 gives the number of minutes you want to descend. That time multiplied by your nm per minute tells how far out you should start. In pressurized aircraft you can choose any vertical rate from 1000 to 2000 fpm.
Once you determine it is time to start down, you reduce power as needed to maintain a safe airspeed while the autopilot brings you down at the set rate. The safe airspeed is normally the speed for turbulence of 2X your clean stall speed. You generally assume some turbulence may be waiting at some intermediate altitude. In a fixed-gear aircraft your transition to the pattern flying is fairly easy. You hold 15 inches map for a while in level flight to get to the speed where flaps can be deployed. You enter the pattern and put first flaps down as you start descending, finishing with full flaps or less as wind dictates.
In a more complex aircraft, you must know and use the speed limits for gear and flap extension. These should be written on your kneepad (or in a notebook) so they are handy if you are not thoroughly familiar with the aircraft. My panels always show gear airspeed with a light and often show flap airspeed with a light. In most cases, first flaps can come out at a higher speed than gear. But in some cases the gear is made very sturdy and should be used first to slow to flap speed. In jets, you must often use spoilers to slow so you can extend first flaps. You use these methods of deceleration to get down to the pattern or the ILS. You must also observe the universal speed limit of 250 KIAS below 10,000 ft.
In piston aircraft, you do not have the luxury of simply pulling the throttle back to idle. That will ruin your engine in a third of the mean time to overhaul. The trick is to reduce power to something like 50% and ride down with closed cowl flaps to retain engine heat as much as possible using a suitably low vertical rate with good planning. In piston aircraft, the only limit you must not exceed is a KIAS limit, VNE or 3X clean stall. Descending from any significant altitude requires attention to many factors in a piston aircraft.
In jets there are more factors. You can normally deploy spoilers at any speed, even beyond Max Mach because that is the only way you can get back below MaxMach (MMO) to save the airplane. All jets are susceptible to MMO excursions during descent while above FL300. You will often need to use the spoilers a little when starting down. Your AP will generally be set for -2000 fpm with thrust reduced somewhat gradually to iidle. Below FL300 you need to worry about airspeed limits, both VNE and VA or max speed in turbulence ( 3x clean stall or 2x clean stall, respectively). Bleed air pressure is normally used to maintain cabin pressure so you don't want to cause cabin altitude to go too high with a sudden drop in thrust. As you pass through FL100 make sure your speed is 250 KIAS or below. Within 10 nm of the destination, you should be starting to use the spoilers, flaps and gear to regulate your airspeed so you are prepared with gear and moderate flap extension when you start flying the approach.
Hans mentioned that he found the Falcon jets do not descend very quickly when the thrust is reduced in level flight. I did a little study and found he is right. For the study I started in steady level flight at 20,000 ft and cut thrust suddenly to zero and turned off the autopilot to allow unlimited descent. The table shows the vertical rate after 30 seconds.
Falcon 50_______________-743 fpm
Citation II_______________-1998 fpm
Hawker 800XP___________-4768 fpm
Hawker 400XP___________-4942 fpm
Learjet 45______________-5776 fpm
Boeing 737-400__________-8852 fpm
In all cases there was very little change in airspeed. After an initial decay it builds back up to the steady value for which the aircraft was trimmed. The aircraft begins a phugoid motion, exchanging vertical speed for airspeed with pitch oscillations while holding angle of attack nearly constant.
But this has nothing to do with flying a proper descent in any jet. In flying the Falcon, I start down with a reduction in thrust, and a change in the autopilot setting looking for a low altitude with a 2000 fpm limit in vertical rate. I generally tap the spoiler key a couple times. The autopilot gets the plane on the desired descent condition quickly.
Any questions?
It might be interesting to discuss this topic. We can use various aircraft as examples. It can be challenging in any aircraft. The basic problem in climbing is that airplanes run out of power with altitude so that they must be flown carefully to maintain a safe airspeed with a reasonable climb rate. In some aircraft a "reasonable" climb rate is anything you can get with an airspeed 30% over stall. After takeoff in any airplane, I turn on the autopilot to make turns toward the departure path and to control climb. I always use the altitude controls (ALT mode) with either HDG mode or NAV mode for steering. In ALT mode your only control is the vertical speed setting. You can start climbing with a good vertical speed but you must adjust this with altitude to keep the airspeed from getting too low. Take the clean stall speed and add 30% to handle wind shear or turbulence. That is your minimum indicated airspeed at any altitude during the climb. in a simple non-tubocharged piston aircraft, you have the additional burden of adjusting the mixture control to maintain max power (or max fuel flow) during the climb. In all other aircraft the vertical speed is your only control.
Descent has some challenges for all aircraft. You have to decide, based on average ground speed and descent rate how far from the destination you will begin your descent. From flight levels above Fl200 the rule is x=3*H/1000. Divide your altitude by 1000 and multiply by 3. In a turboprop or pressurized piston, you'll be at least 60 nm from the destination when you start down. In a bizjet, you'll be 120 to 150 nm from the destination. in am unpressurized piston, you will descend at 500 fpm for ear comfort. So you have to figure 2 minutes per thousand feet. estimate distance based on your average ground speed in n miles per minute. (Divide knots by 60.) Your altitude over pattern altitude divided by 2 gives the number of minutes you want to descend. That time multiplied by your nm per minute tells how far out you should start. In pressurized aircraft you can choose any vertical rate from 1000 to 2000 fpm.
Once you determine it is time to start down, you reduce power as needed to maintain a safe airspeed while the autopilot brings you down at the set rate. The safe airspeed is normally the speed for turbulence of 2X your clean stall speed. You generally assume some turbulence may be waiting at some intermediate altitude. In a fixed-gear aircraft your transition to the pattern flying is fairly easy. You hold 15 inches map for a while in level flight to get to the speed where flaps can be deployed. You enter the pattern and put first flaps down as you start descending, finishing with full flaps or less as wind dictates.
In a more complex aircraft, you must know and use the speed limits for gear and flap extension. These should be written on your kneepad (or in a notebook) so they are handy if you are not thoroughly familiar with the aircraft. My panels always show gear airspeed with a light and often show flap airspeed with a light. In most cases, first flaps can come out at a higher speed than gear. But in some cases the gear is made very sturdy and should be used first to slow to flap speed. In jets, you must often use spoilers to slow so you can extend first flaps. You use these methods of deceleration to get down to the pattern or the ILS. You must also observe the universal speed limit of 250 KIAS below 10,000 ft.
In piston aircraft, you do not have the luxury of simply pulling the throttle back to idle. That will ruin your engine in a third of the mean time to overhaul. The trick is to reduce power to something like 50% and ride down with closed cowl flaps to retain engine heat as much as possible using a suitably low vertical rate with good planning. In piston aircraft, the only limit you must not exceed is a KIAS limit, VNE or 3X clean stall. Descending from any significant altitude requires attention to many factors in a piston aircraft.
In jets there are more factors. You can normally deploy spoilers at any speed, even beyond Max Mach because that is the only way you can get back below MaxMach (MMO) to save the airplane. All jets are susceptible to MMO excursions during descent while above FL300. You will often need to use the spoilers a little when starting down. Your AP will generally be set for -2000 fpm with thrust reduced somewhat gradually to iidle. Below FL300 you need to worry about airspeed limits, both VNE and VA or max speed in turbulence ( 3x clean stall or 2x clean stall, respectively). Bleed air pressure is normally used to maintain cabin pressure so you don't want to cause cabin altitude to go too high with a sudden drop in thrust. As you pass through FL100 make sure your speed is 250 KIAS or below. Within 10 nm of the destination, you should be starting to use the spoilers, flaps and gear to regulate your airspeed so you are prepared with gear and moderate flap extension when you start flying the approach.
Hans mentioned that he found the Falcon jets do not descend very quickly when the thrust is reduced in level flight. I did a little study and found he is right. For the study I started in steady level flight at 20,000 ft and cut thrust suddenly to zero and turned off the autopilot to allow unlimited descent. The table shows the vertical rate after 30 seconds.
Falcon 50_______________-743 fpm
Citation II_______________-1998 fpm
Hawker 800XP___________-4768 fpm
Hawker 400XP___________-4942 fpm
Learjet 45______________-5776 fpm
Boeing 737-400__________-8852 fpm
In all cases there was very little change in airspeed. After an initial decay it builds back up to the steady value for which the aircraft was trimmed. The aircraft begins a phugoid motion, exchanging vertical speed for airspeed with pitch oscillations while holding angle of attack nearly constant.
But this has nothing to do with flying a proper descent in any jet. In flying the Falcon, I start down with a reduction in thrust, and a change in the autopilot setting looking for a low altitude with a 2000 fpm limit in vertical rate. I generally tap the spoiler key a couple times. The autopilot gets the plane on the desired descent condition quickly.
Any questions?