is a flight regime
, winged aircraft
s). This means it describes a particular range of aircraft attitude
s and power settings - which in turn describe a particular performance envelope
Why is it important?
Training for maneuvers in slow flight conditions and gaining experience with the airplane in this regime is critical in order to be able to safely pilot an airplane in normal, everyday use. This is because unlike some other extreme flight regimes (high speed flight, aerobatics, etc.) the airplane will experience slow flight at least twice during every normal flight, and during the two most dangerous times - at takeoff and at landing. At both times, the airplane is in the slow flight regime, although in the first it is under power in order to 'push through it' and the latter it intends to land and hence stop flying.
The most common 'loss of control' event in light airplanes involves the stall. Airplanes in slow flight are at their most vulnerable to stalling, as they have little energy to spare and are at the edge of their total performance envelope - even 'normal' control maneuvers can push them out of that envelope into a stall. As a result, training in slow flight helps the pilot learn to avoid mistakes that would do just that, as well as learning how to recover from incipient (and actual) stalls. Finally, when an aircraft is recovered from a stall, it will be in slow flight initially, so learning how to safely transition out of slow flight is extremely important - not just at takeoff, when everything (should) be operating according to a plan, but after a stall, when things need to happen almost as a reflex in unexpected circumstances.
How to enter slow flight
WARNING: This information SHOULD NOT BE USED in actual flight!
This is a notional set of steps which describe what a pilot 'does' when placing a typical light airplane (such as the Cessna 172 or other high-wing trainer) into slow flight. See the airplane's flight manuals and consult an instructor for your type before attempting slow flight in an unfamiliar airplane!
- Trim the airplane to level flight, at cruise, maintaining altitude.
- Reduce power, while maintaining altitude. Note that the airspeed (IAS) will drop.
- Continue to reduce power. When indicated on the airspeed indicator, add flaps and at some point (above the aircraft's stall speed, assuming you're in clear air, don't worry!) you will find that reducing power any further results in the aircraft beginning to lose altitude, no matter what you do. Add a bit of power, just enough to allow the airplane to maintain altitude. This is probably your airplane's best endurance speed, i.e. it can achieve its best range at this speed. The airplane's nose is likely higher than it was at the start of this process.
- If you reduce power below this point, you will enter slow flight.
Flying in slow flight
Slow flight is sometimes referred to in manuals as 'the area of reverse command.' This is because once you drop below the best endurance speed, you'll find the 'function' of two of your main controls - the throttle and the pitch - have suddenly switched places. Whereas before, in cruise, if you wanted to increase speed you simply increased the throttle (power), that no longer works in slow flight. In slow flight, to increase speed, you lower the nose. In cruise, to increase altitude, you would raise the nose of the airplane. In slow flight, to increase altitude, you increase power.
This reversal is one of the first 'differences' you are taught about flying in slow flight. It can affect you in emergencies as well as in general operations - for example, the aircraft's nose is typically fairly high in slow flight, and it's difficult to see obstructions in front. If something cropped up (another aircraft or a radio tower, say) the pilot might need to suddenly climb. However, the instinctive 'yank back' on the stick would not make the airplane climb - rather, it would slow it down, and likely stall it. The proper response would be to add throttle without changing the attitude, or to attempt a careful turn (again with throttle, since turning increases the effective angle of attack and can also induce stalling).
Slow flight differs in more immediate ways as well, which are important for the pilot to recognize and be familiar with. First of all, as airspeed drops near the stall speed of the airplane, warning systems will probably start to go off (stick shakers or warning horns) and, if flight is maintained, may continue to operate intermittently as atmospheric conditions cause their sensors to bounce in and out of 'danger' readings. Second, the controls of the airplane will be extremely 'mushy.' They will be easy to move, and it will take large movements of the control surfaces to produce the desired effects, because less air is traveling across them due to the slow speed. The pilot will need to become accustomed to the large control deflections required to maneuver. Third, simply maintaining the airplane in slow flight will either require a great deal of constant backpressure on the stick or large trim adjustments - both of which should be experienced by any pilot. While the trim tab adjustment is of course the less tiring way to go about it, it's good to train without that - because if you're in slow flight due to stall recovery or other problem, odds are your trim tab isn't where you'd like it to be and you don't have time to fix it.
Despite all this, slow flight is not inherently dangerous, so long as proper precautions are taken. Modern trainer airplanes like the Cessna 172 are incredibly forgiving, and even if you manage to stall them, recovery can be quickly accomplished. Despite that, slow flight training should always be done with instructor approval, and at an altitude which allows for easy recovery in the event of stall - 3,000 ft AGL at an absolute minimum.
It can even be fun! If the outside winds are strong, a plane like the Cessna 152 can fly essentially forever trimmed to just above its stall speed of around 46 knots. Finding winds aloft of 45-50 knots on a clear fall day is not difficult. Turning directly into the wind and trimming for slow flight will result in, yes, 50 or so KIAS - but a negative ground speed. To the observer on the ground, you will be flying backwards. With practice, you can adjust your speed to 'hover' your airplane over a particular point on the ground if the winds are steady and cooperative (I've done this. It's ludicrous amounts of fun. -tc).
Slow flight can also be used to follow ground vehicles or to make it easier to photograph ground targets from the airplane. Although again, it should only be undertaken at sufficient altitude, I have used it to 'follow' a freight train along the tracks below me just for fun. I find that attempting to follow the train and remain in the same position relative to it is a good practice for learning how to trim the airplane for speeds that you're not used to flying - and a train is big enough to be perfectly visible from 3,000 feet AGL.
So that's slow flight. Don't fear it. Make it something you are familiar with, and it can be your friend. Landings will suddenly become much less stressful, as a slow-moving airplane becomes something you're much more familiar with - and you can pay more attention to the delight that is controlling a feather-light airplane drifting down homewards. Friends want you to do a fly-by slow enough for photos? Slow flight ftw. Finally, it's a great way to sightsee in general, although you probably want to avoid going low enough to trip the warning horns out of consideration for your pax.