Yesterday's (Saturday) lesson was more slow flight, as will the next few. I'll post about the lesson itself separately. The new topic for this lesson was Spiral Dives.
In preparation, I read about spiral dives in both From the Ground Up, and in Transport Canada's Flight Training Manual. I was underwhelming by the explanations in both. So (since I'm blogging for me) I need to make notes about spiral dives. If I can explain it, I understand it.
A spiral dive is really quite simple, and this is why I was probably under-whelmed by the manuals. Entry is not as dramatic or as sudden as, for example, a spin. Recovery is simple. The only real drama is that, because speeds are high, there may be some interesting G-forces involved.
In a spiral dive the aircraft is flying "normally", meaning that the wings and control surfaces are all functioning, nothing is stalled, the aircraft is traveling pointy-end first, and so forth. The aircraft has a steep angle of bank, and is descending rapidly. The airplane is flying in a corkscrew pattern in rapid descent.
The issue is that an aircraft in a spiral dive, if left uncorrected or if corrected improperly, will very quickly overspeed, resulting in damage or destruction of the airplane (and occupants).
Because of the potential for airframe damage or destruction, Transport Canada states that it is not a permitted solo flying activity, and even when flying with an instructor it is the instructor who must put the aircraft into the spiral, and then transfer control to the student for recovery.
Recovery is simple:
- Recognize you are in a spiral dive.
- Close the throttle.
- Level the wings.
- While leveling the wings keep the nose down (it will want to rise due to the high airspeed).
- Once the wings are level, pull back on the stick/yoke. Expect to feel G-forces.
Recognizing you are in a spiral dive is the critical first step, and because of the risk of overspeed it must be done quickly. You know you are likely in a spiral dive when you are losing altitude, and airspeed is increasing. There is no need to detect the steep bank by referencing the Attitude Indicator (and it may be spinning/useless anyway).
Huh? (This was the first under-explained point in the manuals).
If you are going down (fast) then there are four possibilities:
- You're in a wings-level dive. Altitude is decreasing, and airspeed is increasing. But: Because the wings are level and the airspeed is increasing, the nose will want to come up, since a trimmed aerodynamically stable aircraft will want to fly in level flight at a constant speed, and will return to that attitude if left alone. So, left alone, the rate of descent, and the airspeed, will decrease as the aircraft levels and follows the trim.
- You're in a spin. Altitude is decreasing, but in a spin the airspeed isn't increasing (except due to the engine, but you're going to close the throttle).
- You're in a spiral dive. Altitude is decreasing, and the rate is increasing (you're going down faster and faster). Airspeed is also increasing and moving towards overspeed.
- There is one low-probability possibility.... The aircraft is in a wings-level dive, and being held there due to being badly out of trim (the trim is holding the aircraft in the dive). Or the control linkages have jammed. Worry about 1-3 first.
Close the throttle. In a spiral dive or a spin, closing the throttle is the first step in the recovery, so closing the throttle is done automatically (it also does not hurt the recovery options if you are in a wings-level dive, so there is no downside). As soon as there is a realization that you're rapidly descending, close the throttle, even if you are not sure whether you are in a spin or a spiral dive. Closing the throttle reduces the risk of airframe overspeed and engine overspeed.
Level the wings. Be aggressive. A spiral dive must be corrected quickly, so this is not the time for a lazy roll. Even with an excessive forward speed, rolling the aircraft level will not over-stress the airframe. Full airleron deflection is unwise (if faster than Vno), but fortunately it is also not necessary. Coordination using the rudder produces a roll which is both faster, and with no yaw (which means minimized airframe stress).
During the roll keep the nose down using forward stick/yoke. Because of the high airspeed the nose will want to pitch up as the wings level. Keep the nose down. The aircraft is at high speed, and rolling. If the nose is allowed to pitch up then the aircraft will both be pitching up and rolling, and at high airspeed the dual-axis translation results in higher airframe stress.
Pitch up (gently) to a climb attitude. My instruction has been to always return to a best-rate-of-climb attitude after any rapid descent (stall, spin, spiral). The thinking is that you know you have just lost significant altitude, and you may be at a dangerously low altitude. Rather than level off and then start thinking (and your first conclusion may well be "crap, look at those trees right in front of me, I need to get higher now!"), it is a better strategy to recover to a climbing attitude, trade the high airspeed for a rapid increase in altitude, and then apply power as airspeed decreases towards best rate of climb. Once stable in the climb, then there is time to think about other factors (heading, cloud base, etc.). It was pointed out to me that during the flight test it is important to listen to the examiner carefully, and make it clear what the recovery attitude will be. If the examiner insists on level flight, give him/her level flight, so recovery to a climb is not assessed as an error. Or pre-flight clarify that recovery will be to a climb.
What does not work (and what kills pilots): Because the aircraft is descending and in coordinated flight, the natural reaction from a pilot is to pitch up (pull back on the stick/yoke). However, because the aircraft is in a steep bank the effect of the elevator is to tighten the turn, not pitch-up the nose. The risk is that the pilot will panic and pull-back the elevators even harder, possibly damaging the control surfaces... but most certainly not correcting the rapid altitude loss. If no other action is taken the aircraft will auger-in with the the stick/yoke pulled back to the stops and the pilot screaming "WTF is happening? Why won't she pull out?".
In normal flight, entering a spiral dive is inadvertent, never a normal flight maneuver. The typical entry methods are:
- Disorientation when flying in clouds, night, or IFR conditions, with a pilot that is not instrument-experienced. In fact, the normal outcome of a non-instrument pilot flying in instrument conditions is a spiral or (sometimes) a spin. Usually within seconds or minutes.
- Flying in a steep turn, but allowing the nose to drop. Once the nose drops it is near-impossible to recover using the elevator, and a spiral dive will result unless the wings are leveled. NEVER do a steep turn when in the circuit for landing, as there may not be enough altitude for recovery.
- Trying to enter a spin, but the the wings don't stall and a spiral dive results instead.
The critical step in the recovery is recognition that the aircraft is, in fact, in a spiral dive and not just flying in a steep turn, or in level but severe nose-down flight.
The critical action in recovery is first getting the wings level, not first trying to pitch up.
The critical factor in the timing is recover quickly, before the airframe or control surfaces are at risk of damage from overspeed. "Quickly" refers to reaction time, not slamming the control surfaces to the stops.
The DA20 Eclipse can withstand 4.4G - that's a lot of force. Even then, the practical guideline on G forces is that they are limits when it is an expectation that the aircraft will be reused. If it looks like the aircraft is not going to be reused, there are no limits. Do what is required.
Note to self: Even after the reading, pre-flight instruction, recovering 10-15 spiral dives, and the post-flight debriefing, I still didn't understand all aspects of the maneuver. Which is why I tried to explain them on this blog. The benefit (to me) has been excellent. Lesson: Blog about other flight techniques and maneuvers, just to make sure I've learned them. You're welcome to come along for the ride.