I Learnt About Flying From That

I Learnt About Flying From That was the title of a regular piece in RAF Flight Safety publications which, by rehearsing experience, was intended to prevent recurrence.

MCAS stands for Maneuvering Characteristics Augmentation System but it is not clear to me, an old pilot, whether it acts as compensation for, or augmentation to, “maneuvering,” as we shall see below.

The stability of an aircraft is its qualities of returning to a particular condition of flight having been disturbed by something happening other than the pilot making a movement of his controls.  This is a highly desirable characteristic and, in olden day aircraft, happened naturally because the of the design of the cross section of the wing. In simple mechanics, if the weight of the aircraft and the lift it provides for flight always act through the same point on the longitudinal axis all is apparently well.  If the point at which the lift acts moves in relation to the weight (the centre of gravity) then a mechanical “couple” causes the aircraft to rotate about that axis.  This moment causes the aircraft to “pitch” – nose up and down. Normally, the pilot corrects for this by applying a compensation force on the tail of the aircraft which restores the whole system to balance.  If the restoring moment provided by the tail is greater than the upsetting moment then all is, normally, well.  But, whilst an aircraft’s tail does an essential job for control it does almost nothing for efficiency.  Nowadays, in the increasing quest for efficiency, the design temptation is to exploit the full extent of the flight envelope. The deleterious effects of loss of stability in today's advanced designs can be compensated by providing the pilot with automatic systems that protect the aircraft from dangerous divergence following a disturbance.

Back in the days when I was a basic flying instructor I made several demonstrations to show my student the inherent stability of his aircraft and I showed (and made sure they  understood) that if they put the aircraft nose level with the horizon and set the power to the cruise setting, hey presto, the aircraft would settle down to the safe cruise configuration in roughly level flight and at a constant airspeed without further ado.  I made sure that my student could feel where the throttle lever should be on the throttle quadrant without having to look inside at the instruments.  Equally, I got them to recognise the sound the propeller made when it was rotating at 2000 RPM, the approximate setting for level cruise.  When it came to exploring the maximum performance of the aircraft it was necessary for my student to recognise the approach to the stall, the stall and how to recover from a stall.  This was important because a conventional wing delivers its maximum lift force at just below the stalling angle – any less and you will lose out to a more skilful opponent in combat and anymore and your evasive turn will come to a juddering halt in front of your enemy’s guns. We learned to recognise the stall by increasing airframe buffet as airflow began to object to being bent over the aerofoil wing.  Learning to fly “by the seat of one’s pants” was, literally, true.

The Boeing 737 Max is a development of the venerable Boeing 737.  It differed from its predecessors, in particular, because the engines were bigger and mounted closer to the wing to accommodate the extra diameter without touching the ground.  The result of this physical change was to change the moment of the lift weight couple in the pitch up sense.  The Boeing aerodynamicists felt that the change was significant enough to require a bit of extra help for the pilot so they added a new system to compensate for the aircrafts (new) natural handling characteristics.  In essence, the system helped the pilot reduce the risk of stalling by automatically (without the pilot doing anything) bringing the nose down if the aircraft should approach the stall when being flown without the autopilot.  The system was labelled MCAS.  As I understand it, if there is a push/pull battle with the pilot, MCAS always wins.  And again, as I understand it, MCAS uses the same air data as the primary flight instruments – so if the flight data is garbage, the input to the MCAS will be garbage as well.

Now I have flown aircraft with design features that help the pilot out in similar situations but never when those systems have been able to override the pilot input.  In the Victor “V” Bomber there was an unfortunate aerodynamic idiosyncrasy in that as the aircraft accelerated towards the speed of sound, instead of the nose pitching up with increasing speed, as the Mach Number rose above .9 the speed of sound, the Victor would tuck under and accelerate wildly.  To compensate, the designer helpfully provided a device called a Mach Strut which, when selected on (as it was for normal flight) would provide an increasing nose up input to the controls as the Mach Number rose.  Unfortunately, the Mach Strut was prone to run away, at random.  Accordingly, we were trained to recognise the malfunction, hold the very strong forces on the control column whilst deselecting the offending Mach Strut system, after which, the aircraft could be trimmed to fly normally and the sortie continued using the standby system.  In all my flying on the Victor, I never encountered a Mach Strut run away but lots of other things went wrong, some very serious, and I was grateful for the depth of my training when the chips were down.

Of course, I have no idea what caused the recent 737 Max disaster but it is possible that pilot training had something to do with it.  It is reported that the co-pilot had only 200 hours total flying which, in practical terms, meant that if anything serious went wrong, the workload on the other pilot, the Captain, would be significant.  I began by talking about the Maneuvering Characteristics Augmentation System.  It doesn’t look, to me, as if it does much augmentation – more inhibition through compensation.  How design philosophy has changed? Where I came from, the most reliable form of manoeuvre inhibition was the reaction of the (well-trained) pilot.