Tuesday, November 04, 2008

Short Field Operations

This how-to posting is to make notes for future reference, and to internalize my lesson.

The fundamental principal of short-field operations is simple: Don't waste runway.

For short field take-off:
  • Do the math - get out the AOM, calculate density altitude etc, and make sure you have enough runway - you can skip this step if practicing from a long and proven runway, but if there is any doubt then this is a mandatory step,
  • Position the aircraft as close to the end of the runway as reasonably possible - runway behind you is wasted,
  • Pull the stick right back - when you apply power the nose will tend to dip, so this counteracts this tendency. It also will push down the tail, making the brakes more effective,
  • Toe brakes full on,
  • Throttle to full,
  • Check the gauges - make sure the RPM and all gauges are at expected values,
  • Release the brakes, expect the nose to jump left so be ready with right rudder (and, if the nose wheel is castered, unfortunately you might need a touch of right brake until airspeed gets to the point where the rudder is effective),
  • Release the stick to the neutral position,
  • Rotate at Vr (as usual), and climb out at Vx (57 knots in the Eclipse). Get the aircraft off the ground right at the documented airspeeds - if you rotate and then climb at higher speeds, you will have spent more time hurtling down the runway building up airspeed rather than climbing,
  • When any obstructions are cleared, drop the nose to accelerate to Vy (68 knots in the Eclipse).
For a short field landing the secret is in the approach. You can't land an airplane before it is ready , so the trick is to do all that pre-landing stuff before the threshold of the runway. That way, when the aircraft is ready to land you are just past the threshold of the runway rather than 500 feet down::
  • Within the confines of safety, use a slightly slower approach speed - e.g. 55 knots instead of 60 knots, plus add half the gust factor if the wind is gusty. The energy of any moving object is F=MV*2 (mass times velocity squared). Since the mass is constant, an aircraft flying at 55 knots (compared to 60 knots) has 91% of the speed, but only 84% of the energy. If we flew the approach at 50 knots (the AOM says a normal approach speed is 52 knots) then we'd have only 69% of the energy as compared to 60 knots. Whatever energy you carry into the landing is energy you need to shed before the airplane touches down. It takes more time, and more runway, to shed more energy,
  • Use a bit of throttle during the descent. This is a precision approach, and you want to be able to both add a touch of throttle, and take out some throttle, to adjust the descent profile. In a glider we approached with half-spoilers, so we could both add in more spoiler (if long), and take some out (if short) - really important, since we didn't have a throttle,
  • The aiming point is before the runway threshold, as is the flare. The aircraft floats for a while in cruise while you bleed off airspeed, so you do the flare and float before the runway starts, rather than floating over the limited runway you have,
  • There is a bit of throttle during the descent to manage the descent profile, but you want to have the engine at idle by the time you start to flare. Any engine power beyond this point pulls you forward, and lengthens the float,
  • After touchdown, flaps from landing to cruise. You lose some aerodynamic drag by raising the flaps, but it significantly lessens the wing's lift - and brakes are much more effective (tires have much more traction) when there is a heavier weight on them,
  • After touchdown, apply full brakes without locking them,
  • After touchdown, apply back stick pressure to maximize the weight on the main wheels, to manage the descent of the nose wheel to the runway, to minimize the weight on the nose wheel, and to minimize nose-dip (maximize propeller clearance) due to the heavy braking - but without popping a wheelie or jumping back up into the air.

Finally, I'm told that the examiner will declare that there is a 50 foot obstacle somewhere before the start of the runway which must be cleared. This is mostly a mind game - a normal approach will easily clear such an obstacle.

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