A_P_2015_04

April 2015 African Pilot 83

Accident Report

candidate only had 14 hours of flying time within the 90 days prior to
the accident. This means around four hours per month or one hour per
week. This is hardly conducive of building a professional career as
stated. The candidate was hour building towards a CPL and this was
going to take many months. What I would deduce from the minimal
flying hours is that the pilot probably elected to write and get the CPL
subjects behind her and then continue to build her hours in earnest.
This is very normal for most CPL aspirant pilots and something that
requires more attention than we would like to acknowledge.

Here is my problem. The instructor was supposedly invited to ‘baby-sit’
and as such was not paying full attention to the pilot’s actions that is
until it was too late to avoid the stall, but luckily soon enough to save
the day from fatality. So my problem is not actually with the instructor
as he was on board as a passenger. When we invite people to oversee
us and they do not have official status, this normally, or in many cases
until now, proved to be problematic. The instructor was invited to help
the candidate not getting lost and to help with procedures and radio
work, but he certainly did not expect her to get into trouble during the
take-off phase. This would account for his late interference.

Oh yes, I need to address stalling. So, here goes. One definition of
basic stalling is; when an aircraft with engine or engines throttled back
in clean configuration, can no longer maintain straight and level. This
stalling speed is derived directly from the Lift formula; L=CL½ρv²s (Lift
= Coefficient of Lift, half density, velocity square and surface area of
the wings) and because the definition deals only with straight and level,
lift is considered to be equal to weight, until as it states in the definition,
the lift can no longer sustain the weight of the aircraft. Now, any flight
other than straight and level is considered to be manoeuvring and as
such stalling speed when not in straight and level is referred to as
manoeuvring stalling speed; the reason being that in straight and level
the load on the aircraft is one ‘g’. Load is total lift divided by weight,
but for now to refer to it as a ‘g’ force is okay. In any other flight other
than straight and level the ‘g’ will differ. In a turn, a pitch up and so on
it would be positive and as such more than one ‘g’. In a descent, as in
a climb, lift is always less than weight. That means that the load differs
on the aircraft. Stalling speed differs as the load differs in the formula;
VM=VB√n (Vm is manoeuvre stalling speed, VB is the basic stalling
speed and n=“g”). This all means that aerodynamically the Green
Arc stalling speed on your airspeed gauge is only for straight and
level. Whenever the load changes to positive, then the stalling speed
increases. Let us say the stalling speed is 60 knots and you pull four
‘g’, the result would be a stall that will occur at 120 knots. So any hard
pull-up, or hard banking and pull-up during a ‘shoot-up’ is potentially
fatal as the stalling speed of your aircraft in the clean configuration has

just gone up and you do not know how much. Add to this a reduction in

speed coupled to a higher stalling speed and where the two meet, you

flick and fall out of the sky.

To recover from a stall you have to unload. This means you must revert

to less than one ‘g’ as this would firstly reduce the stalling speed to below

the basic or green arc stall speed. Secondly, it would simultaneously

reduce the angle of attack and as such get the wing below the critical

angle. Note that getting the angle of attack less than the critical angle is

not enough as the speed could still be too low, but the load factor would

reduce the stalling speed significantly. For example, the same aircraft as

above will stall at around 43 knots with half a ‘g’ loading.

Power addition is critical to the recovery as it re-energises the inboard

section and alleviates the stall considerably; let us say around five

knots lower with full power. In our case the investigation states that the

propeller markings are indicative of a high power setting. Well, not so,

when the blades bend rearwards and the blades are also not otherwise

bent or twisted, this is a clear indication of no power on the propeller.

In the case we are reviewing, the power was closed prior to impact with

the ground. When this reduction took place is not mentioned, as the

investigator made a fundamental error in not analysing power settings

during the impact sequence.

Please stay with me and understand the following; there are only three

basic energies available for us to fly with. It is Kinetic Energy (½ρv²),

but speed in this formula of KE is the one of interest for now, Potential

Energy (height above ground) and Mechanical Energy (thrust from the

engine). Now, in a stall the KE has dropped to a pretty useless quantity

and needs to be restored. The engine is there to provide thrust, but

if the height energy is too low, meaning you have no height in which

to recover, the die is cast. The bottom line is when there is a need to

recover from a stall we have to use all the energies and a lack of height

to convert to speed is normally fatal.

The recovery from this condition should have been to unload the

aircraft, maintain full power and use the available height. If the power

was closed too soon, a possible recovery was missed. There was

obviously flying speed and the instructor managed to turn the aircraft

and they did not flick or lose control, so the aircraft was flyable. The

landing was actually not bad, except too hard, but still pretty much under

control. I would just like to have known where the power reduction took

place. Well, I will never know as the pilots involved in the scenario are

unknown to me and I actually do not want to know who were involved

in the accident. Please note that the comments are not to condemn, but

to learn from the unfortunate.