There has been no topic more hotly contested in bars and internet fora over the years than “what controls what” on the approach. The question is a simple one; when on finals do I control speed with power and glidepath with stick, or speed with stick and glidepath with power? Happily the answer is relatively simple.
The graph below shows total drag, which is made up of Induced Drag (aka Lift Dependent Drag) and Zero Lift Drag (aka Parasite Drag). Induced drag, being dependent on lift, is higher when more lift is being generated. Thus induced drag increases as the aircraft slows down. Zero lift drag doesn’t care about lift but increases exponentially with increasing speed. The speed at which the sum of induced drag and zero lift drag – called total drag – is lowest is the minimum drag speed, or Vmd.
But why should I care about this, Jim? Good question. The graph is vitally important to understand what speeds to fly at for range, endurance and when the engine stops – but we’ll cover that elsewhere. It’s also key on the approach.
Imagine an aeroplane flying on the “front side” of the drag curve, i.e. faster than Vmd. It experiences some brief, light turbulence which makes the speed drop a couple of knots. This takes the aeroplane nearer Vmd, so the total drag reduces. With reduced drag the aeroplane will speed up again, eventually recovering to the original speed. So when on the “front side” of the drag curve we say the aeroplane has positive speed stability, or is “speed-stable”.
Conversely, let’s now fly on the back side of the drag curve, i.e. slower than Vmd. We hit the same patch of light turbulence and we slow by a couple of knots. This time, the aircraft is moving away from Vmd, so total drag increases. Higher drag results in more speed being lost, increasing total drag giving an even faster rate of speed loss. Clearly, we are in a region of negative speed stability and the aircraft is “speed-unstable”.
Whether your aircraft’s approach speed is stipulated to be on the front side or the back side is largely determined by its role. Aircraft designed for short landings (Cub, Storch, F/A-18) will generally be on the back side. Aircraft designed for long runways or passenger comfort will generally be on the front side. Some will have speeds for both, e.g. normal or short-field landings.
Budding test pilots are taught about the “front side approach technique” and the “back side approach technique”. Let’s deal with the easy case first.
Front side approach technique. We’re in a PA28 Cherokee flying an approach on the front side of the drag curve, so we’re speed-stable. We’re a little low – we pull back on the stick trading speed for height. Now speed stability doesn’t get you free energy – you’ll need to add power to stop the speed from decaying. And once we’re back on the glidepath we lower the nose to point at the runway again, and put the power back where it was. We continue pointing at the runway and, barring any further turbulence, the speed pretty much takes care of itself.
So far, so intuitive. But now for the…
Back side approach technique. We’re in a PA18 Cub flying an approach on the back side of the drag curve, so although we didn’t know it we’re speed unstable. We hit our turbulence and the speed decreases; unlike with our PA28 if we didn’t do anything the speed could continue decreasing to the stall. The quickest way of getting the speed back under control is to use forward stick to lower the pitch attitude. This will put us below the glidepath so we’ll need to make a power increase as well, but this is less time-critical. Once back on glidepath, keep controlling speed with stick and bring the power back to datum. Yes, this technique means we’re constantly adjusting attitude down the approach but this is much more efficient than constantly adjusting power (even more so with a jet engine with its longer time to spool up / down). Speed with stick means we have to use power for glidepath – since glidepath takes longer to correct this is fine.
Let’s look at a couple of opposing use cases to paint the picture.
- Airliners fly front side approaches – minimising throttle inputs increases efficiency, passenger comfort and the extra speed gives an increased margin of safety in case an engine fails. Being speed stable also reduces pilot workload.
- Carrier-borne fighters fly back side approaches since minimising speed at touchdown is vital. Even in a heavy F/A-18 with significant inertia, the aircraft is trimmed for “on-speed angle of attack”. This means that the flight control computers will, without pilot input, actively return to the datum speed (really angle of attack) after a disturbance. The technique, therefore, is not to touch the stick in pitch – only for lateral corrections. Power is modulated frequently to keep exactly on the glidepath (“ball”)1 – note that carrier aircraft engines are designed to respond very quickly on the approach.
So which technique should I use?
Ascertain Vmd for your type (Vmd is best range speed for props and best endurance speed for jets) and determine whether the approach speed you use is above or below Vmd.
- If it’s above, you’re on the front side and the front side technique will be easier, but you can use either technique if you wish.
- If it’s below, you’re on the back side and you should be using the back side technique.
Hopefully that has helped put some queries to rest and, should the discussion in the bar turn to approach techniques you’ll at least have some data to add to the mere opinion of others!