Energy management on final differences question
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Energy management on final differences question
Lately I was asked why energy management on final approach differs so much with basic training aircraft and later techniques on bigger planes like jets.
I tried a few explanations:
Basic, SE prop driven aircraft, flown under VFR conditions will normally be flown with pitch=speed and power=path(descent rate). Gliders are also being flown like this, with the airbrake lever acting as path control.
I think it will start to differ when we use these planes IFR on ILSes, if I recall it from long time ago.
When flying larger planes (at what power level/weight that would typically change?) and of course jets it is pitch=descent rate and power=speed, whether flown VFR of IFR.
I remember from my own career during the conversion to jettraining it cost me some sweat before I got myself reprogrammed to throttle for speed control.
Would there be any (ab initio?)schools that start right away with the power=speed system in order not to have a dip in the training course?
More ideas around this item?
I tried a few explanations:
Basic, SE prop driven aircraft, flown under VFR conditions will normally be flown with pitch=speed and power=path(descent rate). Gliders are also being flown like this, with the airbrake lever acting as path control.
I think it will start to differ when we use these planes IFR on ILSes, if I recall it from long time ago.
When flying larger planes (at what power level/weight that would typically change?) and of course jets it is pitch=descent rate and power=speed, whether flown VFR of IFR.
I remember from my own career during the conversion to jettraining it cost me some sweat before I got myself reprogrammed to throttle for speed control.
Would there be any (ab initio?)schools that start right away with the power=speed system in order not to have a dip in the training course?
More ideas around this item?
It's a popular topic on these boards; this is a reasonable example of a thread. Post 9 makes a good case for using the point + power technique in all aircraft. Do a search for 'Point and Power' to uncover lots more heated debate.
It helps me to think of it like this:
You want to fly your airliner down a constant descent path. You have a ready indication of this in the cockpit - the ILS glideslope - and because of the inertia of your large aircraft, once you have got it descending at 3 degrees, you want to keep it there rather, than chase it all the way. Therefore slight deviations from the glideslope are immediately dealt with using pitch (using power would result in a delay for spool-up or -down, and hence larger divergences from the glideslope). Because you used pitch to correct, you will now get a change in airspeed. You correct this change in airspeed with power. Due to lag in this cycle, the airspeed may wander above and below the target speed by a few knots. However, because airliners fly their approaches with a higher margin above the stall than is the case for light aircraft and gliders, this variation in airspeed is acceptable.
Of course the expert answer is "pitch AND power control speed AND descent angle". However since we are all mortal I offer the following sketch to show the practical differences between the 2 techniques. Deciding whether it is more important to fly a constant approach angle or a constant speed should help you decide which technique is more appropriate:
If you browse through the thread I linked at the top, you'll see mention of the US Navy. They are unusual jet operators in that they don't practice 'point and power'. The reason for this is that when landing on carriers the aircraft must be at exactly the right airspeed, with exactly the right AoA on touchdown or the arrester hook will miss the wires. Because of the need for total precision in AoA, it is directly controlled with pitch. Therefore they must control approach angle with power. They're assisted in doing this by the 'meatball', which is effectively an extraordinarily sensitive PAPI. They do have four wires to choose from, but it's pretty impressive nevertheless!
It helps me to think of it like this:
You want to fly your airliner down a constant descent path. You have a ready indication of this in the cockpit - the ILS glideslope - and because of the inertia of your large aircraft, once you have got it descending at 3 degrees, you want to keep it there rather, than chase it all the way. Therefore slight deviations from the glideslope are immediately dealt with using pitch (using power would result in a delay for spool-up or -down, and hence larger divergences from the glideslope). Because you used pitch to correct, you will now get a change in airspeed. You correct this change in airspeed with power. Due to lag in this cycle, the airspeed may wander above and below the target speed by a few knots. However, because airliners fly their approaches with a higher margin above the stall than is the case for light aircraft and gliders, this variation in airspeed is acceptable.
Of course the expert answer is "pitch AND power control speed AND descent angle". However since we are all mortal I offer the following sketch to show the practical differences between the 2 techniques. Deciding whether it is more important to fly a constant approach angle or a constant speed should help you decide which technique is more appropriate:
If you browse through the thread I linked at the top, you'll see mention of the US Navy. They are unusual jet operators in that they don't practice 'point and power'. The reason for this is that when landing on carriers the aircraft must be at exactly the right airspeed, with exactly the right AoA on touchdown or the arrester hook will miss the wires. Because of the need for total precision in AoA, it is directly controlled with pitch. Therefore they must control approach angle with power. They're assisted in doing this by the 'meatball', which is effectively an extraordinarily sensitive PAPI. They do have four wires to choose from, but it's pretty impressive nevertheless!
Last edited by Easy Street; 9th Feb 2013 at 22:45.
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DESCENT MANAGEMENT
Good descent management is a key factor in operating the aircraft efficiently. It is also an area which has the potential to cause problems for pilots converting to type, particularly if their experience has been attained only on smaller aircraft (with less momentum) or turboprops (which can usually descend and decelerate better).
If we firstly compare energy of an Airbus 320 compared to a small aircraft. Kinetic energy can be measured in terms of ½ × mass × velocity ²
Airbus 320 at a typical descent speed of 270kts at Max landing weight:
66 Tonnes at 270kts = 2,405,700 units of kinetic energy
A typical light aircraft descending at 100kts:
1 Tonne × 100kts = 5,000 units of kinetic energy.
As can be seen from the above calculation, the airbus has approximately 480× the kinetic energy of a light aircraft in the descent. This energy (or momentum) manifests itself in the descent and deceleration to approach and needs to be handled carefully.
It is often commented on that "the aircraft doesn't go down and slow down!", although this comment isn't strictly correct, it does perhaps demonstrate the importance of proper descent planning and management.
Good descent management is a key factor in operating the aircraft efficiently. It is also an area which has the potential to cause problems for pilots converting to type, particularly if their experience has been attained only on smaller aircraft (with less momentum) or turboprops (which can usually descend and decelerate better).
If we firstly compare energy of an Airbus 320 compared to a small aircraft. Kinetic energy can be measured in terms of ½ × mass × velocity ²
Airbus 320 at a typical descent speed of 270kts at Max landing weight:
66 Tonnes at 270kts = 2,405,700 units of kinetic energy
A typical light aircraft descending at 100kts:
1 Tonne × 100kts = 5,000 units of kinetic energy.
As can be seen from the above calculation, the airbus has approximately 480× the kinetic energy of a light aircraft in the descent. This energy (or momentum) manifests itself in the descent and deceleration to approach and needs to be handled carefully.
It is often commented on that "the aircraft doesn't go down and slow down!", although this comment isn't strictly correct, it does perhaps demonstrate the importance of proper descent planning and management.
Basic, SE prop driven aircraft, flown under VFR conditions will normally be flown with pitch=speed and power=path(descent rate).
I suppose it ultimately depends on where the new flyer is headed; for bigger and better things? Train them to fly like aeroplanes are supposed to be flown: pitch controls flight path and power controls speed. Staying a lighty pilot? Do whatever suits.
Gliders are a totally different kettle of fish: they haver no power so much use pitch for speed control.
They are unusual jet operators in that they don't practice 'point and power'. The reason for this is that when landing on carriers the aircraft must be at exactly the right airspeed, with exactly the right AoA on touchdown or the arrester hook will miss the wires.
Last edited by Capn Bloggs; 10th Feb 2013 at 01:19.
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If I have to fly an ILS approach in a PA28, I fly it at cruising speed & use pitch to stay on the glideslope; only reducing to approach speed & configuring once visual.
