Stalling the Tiger Moth

This weekend I got to experience the stall for the first time, and it wasn't as scary as I thought.

After a few weeks of bad weather, I got to the airport slightly afraid I wouldn't even be able to remember which end of the aircraft was the front. But once I was in the cockpit it all started to feel natural again.
We took off and I climbed us out to the local training area at 3000 ft, where'd we'd follow on directly from the previous lesson's introduction to slow flight. Stalling counts as an aerobatic manoeuvre, so we had to do some extra checks this time. Aerobatics has been in the news lately, so I'll return to that another time.

What is a stall?

Some people I've spoken to have been confused about what stalling an aircraft actually means. Sometimes they think it's like when the engine of your car stalls. In fact, it's nothing to do with the engine at all. (Note: existing pilots might want to skip down to the next heading, as none of this explanation is specific to the Tiger Moth.)
Diagrams showing airflow over a wing cross-section at different angles of attack
The airflow becomes more turbulent as the angle of attack increases. Public domain image courtesy NASA.
In normal flight, the air flow over the wings creates lift. The amount of lift depends on the airspeed, and the angle of attack of the wings: how much they're angled up from the airflow. Increasing the angle of attack by pointing the nose up increases the lift—up to a point. When the angle of attack is too great, the airflow over the wing becomes turbulent, and it stops producing lift altogether. When this happens, the wing is stalled. Obviously that's a bad thing, because nothing is holding you up any more, and the aircraft falls out of the sky.
As you fly slower, you need to increase the angle of attack (by pitching up) to produce enough lift to keep the same altitude. Eventually, the angle of attack is steep enough that the wing stalls, and you start to fall out of the sky.
When this happens, you can't use the ailerons to roll the aircraft any more. They work by changing how much lift the wing produces, so if it isn't producing any, they don't work. Trying to roll the aircraft is likely to just put you into a spin.
Each aircraft has a published stall speed, which is the speed where it stalls, if the aircraft is flying straight and level at its maximum weight. But because stalling is caused by angle of attack, not speed, you can stall at higher speeds than this if the angle of attack is greater, e.g. during a steep turn or when climbing.

Slow flight

Last time, we practised slow flight at 65 mph, which isn't terribly slow. This time, we'd go further, to 50 mph. As before, I lowered the throttle to around 1600 rpm, and progressively raised the nose to keep our altitude as we lost speed. Because of that increase in drag, I had to add a little more power as we slowed, so our speed wouldn't keep dropping to the Tiger Moth's stall speed of 30 mph.
Down at 50, the controls were noticeably less responsive. It took a lot more movement of the stick and pedals to have any effect. It took a bit of experimenting, and about 300 ft of lost altitude, to learn that the correct attitude at this speed is the same as for climbing. When I tried to trim for this attitude, I found I had to pull the trimmer almost all the way back before the aircraft would stop trying to dive.
Even so, it was quite easy going once I'd got everything trimmed. I just had to make sure to stay alert and avoid the attitude or airspeed changing. But I didn't have long to relax: I had to recover to a normal cruise, and perform another clearing turn. This time, we saw some traffic (my instructor spotted it first): a Cessna 172 flying much lower than us at 2 o'clock. It was going straight past, and wasn't in our way, so I unlocked the slats and flew the manoeuvre again.
This time, I knew what attitude to aim for, so it was much quicker to get level and trimmed at 50 mph. The controls were just as sluggish, but as the speed decreased, I could see the slats popping out in front of the top wing. With this extra protection, the instructor told me to fly a turn. Because banking to turn increases the angle of attack and brings you closer to the stall, you can only fly shallow turns at low airspeeds. Even getting a 15° bank angle needed me to pull the stick right over, because the controls were so sloppy.

Recognising the stall

In my next lesson I'd have to stall the aircraft, to practise stall recovery, so in the remaining time the instructor elected to show me what it was like. Because I'd read my textbook before the flight, I knew the common signs by which you recognise a stall:
  • Some aircraft have a "stall warning", which buzzes or sounds a horn when your angle of attack is too steep
  • Vibration, shaking, or buffeting of the airframe
  • The altimeter whizzes around anticlockwise as you lose altitude
  • The aircraft becomes laterally unstable, so any imbalance will cause one wing to drop, which eventually becomes a spin
We already had the trimmer all the way back to keep the aircraft in the nose-up attitude of slow flight, so my instructor took control and pulled the stick back. I locked the autoslats closed again, so they'd stop protecting us from the stall. Though the controls were still quite unresponsive, it raised the nose a little, but it seemed at first that nothing happened. The Tiger Moth didn't really show any of these signs. As a training aircraft, it's designed to have quite a benign stall. No buffeting, and not much of a spin tendency, just a flat descent. When I looked back inside the cockpit at the altimeter, it wasn't even winding anticlockwise very fast: maybe 50 feet per second.
It didn't even feel like being in freefall. The large wing area of the Tiger Moth creates a lot of vertical drag as it falls, so the aircraft was still holding us up in our seats. The sensation of falling was less pronounced than being in a descending lift. I've heard of incidents where the pilot did not realise the aircraft had stalled, but never understood how you could fail to notice. (The most famous example is the Air France 447 disaster, where a crew of three left the aircraft stalled for three and a half minutes, while it fell from 38000 ft into the ocean.) After seeing just how gentle the experience was, it's a lot more understandable now why, with enough height, the stall is only dangerous if you don't realise it's happening.
After a short time of experiencing the stall, the instructor showed me how much he was having to hold the stick back to keep us stalled. To recover from the stall, he simply allowed the stick to come forward. The horizontal stabiliser and elevators are designed to remain unstalled when the wings stall, so the elevators still work normally, and allow you to recover. With us pointing nose down (into the direction we were moving), the airflow over the wings was straight again: the angle of attack reduced to below the stall angle. The wings started to produce lift again, and when he pulled back on the stick, we were flying level once more.

Traffic on downwind

We stalled a few more times, and then it was time to return to the airport. My instructor contacted the tower for me, but left it to me to command the flight back: not just working the controls, but looking out for traffic, setting the correct course, and turning at the right times. Because we were coming from the south (the side of the runway the circuit is on), the tower cleared us to join the circuit on the downwind leg, which meant I made a right turn onto that leg when we got close enough.
Almost right away, they came on the radio again to warn us of traffic ahead of us: a Cessna 172 turning onto base leg. We didn't see it at all, so the instructor radioed back to request a right-hand orbit for separation. That is, we'd make a full 360° turn to the right, both to give us more time to see it, and to give it time to get further away from us if we didn't. I carefully made the turn, looking out of the cockpit the whole time to keep an eye out for this Cessna, only glancing in now and then to check I wasn't losing any height. We still didn't see it, but at least it was well away from us by the time I straightened up back onto the downwind leg.
At the right time, I turned onto base leg, and then idled the throttle to begin the descent. Then I was on the lookout, keeping an eye on the runway end so I could begin the turn to final. It's tricky to know when to start the turn, because you have to end up exactly in line with the runway, and I'd never managed to get it quite right before. It's better to be too early than too late, because you can always make a slightly gentler turn by reducing the bank angle, but you shouldn't increase the bank angle: too steep a bank risks stalling when you don't have enough height to recover. I made a little use of this trick to get myself lined up, but came out of the turn spot on.
Previously, I'd had trouble lining up on a landmark to judge the rest of the approach after getting too low to see the runway properly, but this time I was ready for it. I was lined up with a tree behind one of the cabane struts, and focused on keeping it there as we continued descending to the runway threshold. It wasn't until we were a few feet above the runway markings that the instructor took back control and flared to complete the landing. In the debriefing afterwards, he complimented me on how precise and controlled my flying was that day.

Although the stall was a lot less thrilling than I expected, it was great to actually experience it for the first time. Now I'm ready to practise stall recovery myself. But even though slow flight and stalling were the main topics of the lesson, the big achievement of the day was making an accurate approach to landing for the first time.

  • approach to landing was much better this time 
  • having to orbit on downwind to spot traffic

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