Weekly Flight Review - Stalling

For the last week of National Safety Month, we are focusing on everyone’s favourite manoeuvre, stalling. Most of us do stalls because we have to as part of our training and then when we do a BFR but why can’t we practice them more often, so we are more comfortable with them?

If you are uncomfortable with stalls or have not practised these in a long time, RAAus recommends contacting your local instructor to review this lesson!


Why is stalling so important?

Many of the Loss of Control accidents and incidents we see are stall/spin accidents. This shows that there is a lack of understanding of why and how an aircraft stalls. In this week’s review we want to remind members of why an aircraft stalls and then go and refresh our memories on this so that if we find ourselves approaching an inadvertent stall we can recognise and recover hopefully prior to or at the point of the stall. 

 

Why does an aircraft stall?

Definition(s) of a stall:

  1. a reduction in the lift generated by an aerofoil when the critical or stalling angle of attack is exceeded.
  2. Where any increase in the angle of attack produces a decrease in lift.

 

 Both of the above definitions refer to an Angle of Attack.


Let’s revise what the angle of attack is:

 

So, the Angle of Attack or AOA is the angle between the relative airflow and the chordline of the aerofoil.

 

Lets look at a graph showing at what point an average training aerofoil stalls at: 

So effectively we can see a small training aircraft has a representative stall angle of attack of around the 16 degree mark. This angle is sometimes referred to as the Critical Angle or the Stalling Angle.

This brings us to another point and probably the most important point when it comes to stalling:

We always stall when the Angle of Attack exceeds the Critical Angle of Attack irrespective of the speed!

 

So why do we talk about stall speed then??

Manufacturers need a basis to tell us when the aircraft is going to stall. This speed is stated in the Aircraft Flight Manual AFM or the Pilots Operating Handbook POH. Anything that differs from the below criteria will change when the aircraft is going to stall.

- Straight and Level                  - Aircraft at MTOW

- Flaps / Gear UP                     - Most forward aircraft Centre of Gravity (CofG)

- ISA conditions                        - Power Idle

Essentially what the manufacturer is saying is in straight and level at 1G this is when you expect the aircraft to stall. If not at 1G or in straight and level this speed will change. Remember it’s the exceedance of the Critical Angle of Attack as to why an aerofoil stalls.

  

Stalling Flight Review

Stalling Symptoms

The symptoms of a stall are similar in most training aircraft and are very important to recognise in the hope we can recover even before the stall has occurred.

Some of the symptoms of a stall are:

High Nose Attitude – remember a high nose attitude does not only mean it is pointed towards the sky. Imagine you are in a dive and pull back hard on the control stick, you are going to have a higher nose attitude than expected. Remember the attitude is generally linked to the Angle of Attack of an aerofoil.

Low and Decreasing Airspeed – In straight and level 1G stalls we are practicing generally we have a slow and decreasing stall speed HOWEVER as we have already stated we can stall at any speed.

Less Effective Controls – In straight and level 1G stalls due to a low and decreasing airspeed and therefore less airflow over the flight controls we will have to move the controls more.

Light Control Buffett – Due to separated turbulent airflow impacting elevator control surface

Control Column Position – Probably the most important point when it comes to the symptoms of a stall in the position of the control column (stick or yoke). An aircraft is not likely to stall itself, so the big problem is the pilot pulling back too hard of the control column and therefore allowing the aircraft to exceed the critical angle of attack. Take particular note of where the control column is, measured aft from the dashboard, when the aircraft stalls. In ‘most’ light aircraft the control column is almost on the rear elevator stops when the aircraft actually stalls.


Revision of Slow Flight

Before conducting the stalls we suggest you set up the aircraft for slow flight. You should be able to start seeing the beginning of the stall symptoms. It is important to feel how the aircraft feels at the slower speed in terms of controllability. Once comfortable with how the aircraft reacts and feels you can proceed to conducting a stall. 

 

Pre-Stalling Checks

It is important when conducting the stalling exercise that we do a pre-manoeuvre check prior to stalling the aircraft. A generic one that covers all the basics is the HASELL check:

Height – sufficient height to recover by 3000ft AGL

Airframe – configuration of the aircraft for stalling (i.e. flaps & gear up) & aircraft approved for stalling

Security – no loose items in the cockpit. Even a paper checklist floating through the cockpit can distract you when trying to recover from a stall

Engine – Temperature and Pressures in Green, Fuel Pump ON, tank selected. Think about the engine after a long climb on a hot day or large power changes on a cold day

Location – Not over any populated areas, not close to any airspace or cloud that you may inadvertently enter, plenty of landing areas below if required

Lookout – Complete a clearing turn of 360° to ensure airspace is clear prior to stalling

 

Conducting the stall

  1. Pick a reference feature in the distance to assess any deviations and a reference altitude
  2. Carburettor heat – HOT (as applicable)
  3. Power to idle – smoothly to idle & balancing with rudder
  4. As aircraft slows down apply backpressure to maintain height until aircraft stalls (notice symptoms and maintain in straight and level)
  5. Carburettor heat -COLD (as applicable)


Recovery from the stall

As discussed, we are stalling because the angle of attack has exceeded the critical angle of attack. It has nothing to do with the speed. Our main aim is to reduce the angle of attack to below the critical angle of attack.

  1. At the point of the stall the nose will drop (might be very minor in some aircraft and more drastic in others) and we will start descending.
  2. Release backpressure to ‘break’ the stall
  3. Maintain directional control with rudder. If a wing drops apply oposite direction rudder - Do NOT try to raise the wing with aileron as this will increase the angle of attack further stalling the wing.
  4. Once in a dive, recover from the dive

We have not mentioned use of power here. Power is used to help minimise height loss following a stall but it is important to remember that it is a secondary action. We need to release backpressure and then add power as required. Be careful adding power too soon as it may make the stall worse! 

We have only focused on basic stalls in this week’s flight review. We remind members that there are more advanced stalls (with flap & power and from different phases of flight as well as spins that can be undertaken or practiced in aircraft.

If you are uncomfortable why not take an instructor up and get them to help you with your stall revision.