This is an old hairy hoary one!
Perhaps the best way to sum it up is this:
1. When
power is fixed
elevators control
speed; 2. When the
flight path is fixed
Power/
Thrust controls
speed In case 1 consider climbing at climb setting which is fixed. The only way to control speed is by changing pitch attitude with the elevator.
Similarly when descending on idle Power/ Thrust speed is controlled by attitude adjustment with the elevators.
In case 2 consider the condition of level flight and also when following a fixed approach angle whether it be by ILS or visually to a runway aim point. In both these cases the desired flight path is fixed and is controlled by elevator and speed is maintained by appropriate thrust adjustments.
Suppose you are on approach speed on slope on an ILS Glide path (same as a fixed angle visual approach)and the speed suddenly drops 5 kts. There will be a tendency to sink below the glide path. This is immediately prevented by elevator to maintain on glideslope and power is added as generously as necessary to regain desired speed. If on the other hand you lower the nose to regain speed you go below your desired glideslope and now you have two things to set right instead of one. You will still need to increase Power/Thrust. In the meantime you have allowed a higher sink rate to develop close to the ground which may be difficult to correct before ground contact.
On approach to land it is usual to carry an additional speed increment to allow for variations in windy and turbulent conditions this speed is for just such a purpose and should be "used" on a short term basis to achieve a stable approach path.If you make no claim on this extra speed increment it is equivalent to paying for insurance and not making a claim.
[This message has been edited by pterodactyl (edited 29 August 2000).]