Rotorcraft

Another excellent article from the team at Pilot's Who Ask Why - this time on Birdstrikes.

https://pilotswhoaskwhy.com/2024/05/05/the-threats-of-bird-strikes-and-…

Time for Tea and Biscuits:
 
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This tea and biscuits article is for all those pilots out there who think that they do not need to keep their hand on the collective lever at all times during the flight (I am not talking about platforms with autopilot selected).
There will be occasions when you will have to take your hand off the lever to carry out essential tasks.
However, once the task has been completed, the hand should immediately return to the collective.
The collective lever is a primary flight control and, in a lot of cases, also includes the engine primary control – the throttle.
 
There are two main areas that we need to consider:
Pilot reaction time:
The time it takes for the pilot to recognise a problem and react to it.
Intervention time:
The amount of time available to the pilot before the situation becomes unrecoverable.
 
Let’s look at these in a little more detail:
Pilot reaction time:
This is a measure of the speed with which a pilot responds to some sort of stimulus.
Reaction time should not be confused with reflex; they might seem similar but are quite different.
The human has various sense organs through which they feel and responds to stimuli.
The response can be involuntary or voluntary.
Reflexes are involuntary – they are there to protect the body and are faster than reactions.
In a normal reaction, the sensory nerves carry the information to the brain which evaluates and sends a message to the motor nerves to respond to the stimuli in a particular manner.
This is not the case with reflex action.
Although sensory nerves are still involved, they carry the message only as far as the central nervous system, and then quickly back to the motor nerves.
This has bypassed the brain and therefore, you do not have to spend time thinking about the stimuli!
So, a reflex is faster than a reaction.
It should be noted that the human will react to an audible stimulus quicker than to a visual stimulus.
 
Reaction time depends on various factors:
Perception:
Seeing, hearing and feeling the stimuli with certainty.
Processing good reaction time requires the pilot to be focused and have good understanding of the stimulus.
Processing will be slowed down by distraction but speeded up by training.
 
Response:
Motor agility – the ability to be able to move the body quickly, which also requires strength and coordination.
This too can be improved with training.
 
For example, the pilot’s response to an engine failure will be a reaction response (the brain is involved) and not a reflex response.
Training will help to speed up the response by speeding up the processing and increasing motor agility.
So, if we look at the first indication of a sudden and complete loss of power (an engine failure) which will be an un-commanded yaw, its direction dependent on the direction of the main rotor rotation.
The eyes will pick up the visual stimulus (yaw).
The response will be voluntary; it requires the brain to process the information and come up with a response, then to stimulate the motor nerves to move your feet to correct the yaw.
The brain will also be cross-checking memory to see if this stimulus has been associated with any other response requirements (from training), and in the case of an engine failure the brain will link the stimulus to a further response requirement and will stimulate the motor nerves to cause your left arm to lower the collective lever.
This all takes time.
 
Now, if your hand is not on the collective lever at this point then the brain has further work to do.
It must determine the position of the hand and determine from memory the location of the collective lever in relation to your hand position, then stimulate the motor nerves to move your arm/hand from its current position to the collective lever, then further stimulate the arm to lover the collective lever.
This requires a further increment of time, the length of which may partially depend on why your hand was away from the lever.
 
Distraction
Will slow down brain processing.
What level of pilot distraction is there at the point of engine failure?
 
Firstly, is the pilot looking out?
If so, then the visual stimuli will have good ‘certainty’ which will result in quicker processing and therefore a quicker response.
If not, then the stimuli will be the pilot’s muscles feeling a change in ‘G’ forces acting upon them.
Without visual stimuli to add certainty, the brain has a lot of extra processing work to do to come up with a response.
Does it have memory of such a situation?
If it does, then this will speed up the response.
If not, the response will take longer.
 
Secondly, what workload is the pilot’s brain under when the engine fails?
Is the pilot manually changing a radio or transponder frequency?
Are they programming the navigational system?
These tasks are a greater distraction if the pilot is unfamiliar with the task.
Is there stress involved because the pilot is lost or unsure of their position?
If stress is involved then working capacity is reduced further, increasing the processing time and in turn the response time.
Fatigue, hydration, health, fitness, stress levels, training experience, currency in emergency actions, alertness, phase of flight, personality traits etc. will all affect perception, processing and response.
Given the above, we can see that there will be a time increment involved before the collective lever is lowered in response to an engine failure and the response time will depend on many factors, some of which I have mentioned above.
 
The question is, what is going to keep the main rotor RPM above stall during this event?
 
 
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Stored energy.
When the engine stops, the pilot has three available areas of stored energy:
Kinetic energy stored in the main rotor system
Kinetic energy stored in the forward speed
Potential energy stored in the height above the ground.
 
Two of these areas of stored energy are immediately available and one may possibly become usefully available, subject to the correct and timely use of the other two.
 
Intervention time:    
At the point the engine stops, the energy being used to drive the main rotor blades/drive train, is the kinetic energy stored in the main rotor.
The amount of kinetic energy available will depend on the aircraft type.
In all cases, the main rotor will be using the energy stored within it, and as the energy is depleted the main rotor RPM will be reducing; the speed of the reduction in the RPM being dependent on the amount of kinetic energy stored, plus a few other factors like aircraft weight (power being used at the time).
The heavier the aircraft, the more lift required to keep it airborne, therefore the greater the main rotor collective pitch angle required to produce the lift.
High main rotor blade collective pitch angles require more energy to keep them rotating (what I call the paddle effect); therefore, they will use the stored energy quicker, giving a faster reduction in rotor RPM than a lighter helicopter.
Very quickly the stored energy in the main rotor will be sufficiently depleted to cause the rotor blades to stall.
Once the main rotor is stalled, there is no way of un-stalling it.
The time it takes for the main rotor RPM to decay to the point at which the blades stall is the amount of time the pilot has to prevent the stall…which is the pilot’s intervention time!
There is also a time increment required for the aircraft to change from level flight to a descent, which will change the induced flow through the disc from down through the disc, to up through the disc. 
Thus enabling the main rotor to use the potential energy (which is now converted to kinetic energy) stored in your height above the ground.
During single engine helicopter type certification, the manufacturer will have to produce a ‘height velocity diagram’ which takes into account the various areas of stored energy and the average pilot’s reaction time to an engine failure.
The test pilot will, in certain flight configurations, be required to wait one second before lowering the collective lever in response to a simulated engine failure.
 
Energy transfer:
The use of aft cyclic (flare) as part of the response to an engine failure should be taught i.e. the transfer of airspeed kinetic energy to the main rotor to help to maintain sufficient RPM during the entry and establishing an autorotation.
The amount of stored kinetic energy is obviously dependent on your airspeed. The more airspeed the more energy you have available, which is instantly available (flare) and the amount of stored airspeed energy in the Robinson product is greater (unless for example you are flying an R22 below 30kts) than that stored in the rotor system…so understand this and use it!
 
In the days of long hot summers and crispy bacon, when I was conducting the flight element of the Robinson European pilot’s flight safety course, I would demonstrate this transfer of airspeed energy to the rotor system.
The exercise, having first been extensively briefed in the classroom, would be with the student off the controls during the first demonstration, followed by a second demonstration with them following through lightly on the controls.
I would establish straight and level at 2000 ft AGL and 80kts IAS and then demonstrate this energy transfer by rolling the throttle closed and applying aft cyclic to maintain rotor RPM for several seconds before lowering the collective lever and establishing an autorotation.
It must be made clear that this technique of applying aft cyclic does not change the intervention time which is the time it takes to recognise the problem and react.
 
What should come out of reading this article is the fact that the pilot requires a reasonable amount of time to respond to an engine failure; it’s a human factor.
Also that the intervention time reduces dependant on the total weight of the aircraft and therefore the amount of power being required at the point of the engine failure.
 
A pilot flying around with their hand away from the collective lever is not fully in control of the aircraft…so don’t do it!
 
Knowledge is flight safety helping to keep your RPM in the green.