‘There is no such thing as a stupid question’ is a statement I often make while talking to the public about flying, or when briefing students during a simulator session or an elementary training flight at Lanseria. Well, I lied.
There is no shortage of stupid questions – but everything is relative. A lot depends on your mood as to how you respond to what would be more common sense than a technical aspect requiring an in-depth explanation.
“What do you guys do, apart from watching the auto pilot do all the work?”
Well, exactly that. We simply ease back in our seats, order a cup of coffee and watch the autopilot check the weather, read up on all the NOTAMs (Notice To Airmen) that will affect our route, do the flight planning, decide on the fuel uplift, conduct the external pre-flight inspection of the aircraft and complete the entire cockpit set up (all those buttons and knobs on the panel and the overhead console are just for show, you know).
Then we sit enthralled as the auto pilot calls air traffic control (ATC) for the departure clearance, which it then programs into itself and checks if everything is correct and that the gross-error check computes with the fuel on board.
The way it communicates with the ground engineer is a thing of beauty, and the push-back and engine start is handled perfectly.
The decision-making on the ground and in the air is flawless, and the gusty cross-wind automatic take-off complies exactly with prescribed Airbus procedures.
I could go on, but I trust the irony is evident…
To be kind to the originators of the automation-based questions, a peek into the cockpit when the aircraft is straight and level in the cruise may give the impression that we are relaxed and shooting the breeze with each other while the automatics take care of things.
I’ll attempt to summarise the purpose of an autopilot in a jet transport aircraft – or any complex aircraft for that matter. Generally, there are two separate autopilot systems in an Airbus and the narrow-body Boeings. There may be up to three fitted to the wide-bodied Boeings.
They are capable of being engaged by the flight crew when required after takeoff and before (or after) landing. They primarily carry out the stick-and-rudder functions, provided the crew direct it through physical inputs to the auto flight system or through the Flight Management System (FMS), which provides a huge amount of information to an array of aircraft systems, one of which is the autopilot.
So, when we need the basics taken care of, we engage an autopilot with the appropriate mode relevant to the phase of flight. If we have any change to deal with, we must update what we want the autopilot to do, manually or through the FMS.
Unsurprisingly, it cannot understand English and it cannot think or make decisions. That bit is for us ‘underworked’ aircrew.
The fact that all transport category aircraft have autoland capability must surely be proof of the fact the autopilot is able to replace the flesh-and-blood pilot?
The autoland process places a huge amount of pressure on the crew, as opposed to relieving them of it. The fact that we are expecting it to do something out of limits at any stage of the approach keeps us on our toes, and we have to watch it like a hawk, hands and feet on the controls in anticipation of having to intervene at the last moment.
This intervention can take the form of disconnecting the autopilot and landing manually, or disconnecting and commencing an immediate go-around – neither of which are a lot of fun. I’ve had to do a few of both in the past.
Probably the most cynical questions in this regard come from those with a little bit of flying under their belts, normally in simple light aircraft, where the concept of ‘flying’ consists of moving the control column, rudder pedals and throttle. Dynamic decision making is something that’s been read about on the interweb, and the most demanding system related requirement is remembering to change fuel tanks every now and then.
I have been regaled by a private pilot licence holder about how superior his stick-and-rudder skills are compared to the typical airline pilot as he does all the flying all the time during those 50 to 80 hours he does annually. Not a particularly bright statement.
The average airline pilot would do between 750 and 1000 hours annually, in all weather conditions, up to maximum crosswind limitations, onto wet and contaminated runways. This would be apart from the 16 hours of annual recurrent simulator training that covers all aspects of manual flying, interspersed with major failure scenarios and decision-making assessments.
My PPL friend could not remember if the Cherokee he flies has a cross wind limit, let alone what the actual figure is.
On another subject – I have been accused of turning down the oxygen to save money in the cruise. That’s right. That giant oxygen cylinder we have on board to supply hundreds of people with breathable air for hours on end…sorry – I’ll try to stop the sarcasm.
The air that we all breathe (yes, the cockpit ventilation is part of the same system as the cabin – we are not special) is actually the outside air that we encounter up at 39,000 feet. It is the same air that we breathe when we are chilling on the beach, just there is a lot less of it up at altitude. More specifically, it is a lot less dense, which means the nitrogen and oxygen molecules occupy a much larger volume – or are more spread apart – than at sea level.
The magic that sorts this out is called the pressurisation system. This cunningly scoops outside air into air conditioning packages (‘packs’ for short), and pumps this into the air-tight structure of the fuselage. As physical airflow is vital for survival, there is a device called an outflow valve, usually near the rear of the cabin, which for most of the time is in the mostly-closed position to force the air inside to build up to a pressure equivalent to being a little higher than Johannesburg – around seven to eight thousand feet above sea level.
The most critical single-point of failure in this system is the outflow valve itself. If this device ceases being controlled by the automatic cabin pressure system, it will default to the second system. If this one goes on strike as well, we then directly control it from one of those many knobs on the overhead panel (Yup, I lied – they’re not really for show) in manual mode.
Any one of these three systems would allow us to complete the flight. If the manual system fails, then we are in for a depressurisation and the passengers get to grab a mask as the cabin turns into a rubber jungle. The rest is as per that safety briefing to which no-one listens.
Where we do allow economics to get involved, is when we are below a certain passenger occupancy level, the pack flow rate is marginally reduced which puts less demand on each pack. This results in a 0,2% improvement in fuel burn. So yes, we do reduce the flow rate, but no-one would know the difference in the back – just slightly less breezy.
The packs are powered by compressed air (bleed air) which is taken from a specific stage of the compressor section of the jet engines. This in turn slightly reduces the efficiency of the engine, which is why we switch off the air conditioning for take-off.
Turbulence: “We dropped hundreds of feet – I thought we were going to die!”
Possible, but highly unlikely. There are a few different types of turbulence. The most common that is felt on just about every flight is the mechanical and thermal turbulence caused from wind and temperature variations from ground level up to around ten thousand feet. This is more pronounced during summer and is generally what gets passengers throwing up when flying in a light aircraft.
Higher up, we can encounter Clear Air Turbulence (CAT), which is exactly that. Nothing visible either outside or on our weather radar but it can be quite uncomfortable.
Around ten years ago, while over the Atlantic Ocean, we encountered moderate CAT. The descriptions of turbulence are mild, moderate and severe. Mild may spill the coffee. Moderate gets things moving around the cabin and cockpit with some enthusiasm. Severe can result in permanent structural deformation (wing falls off).
In this case, we had significant movement of items in cockpit, such as the contents of my flight bag were flying around, and an apple hit the overhead and whacked down on my hand on the thrust levers.
This continued for around 30 minutes, despite several changes of flight level, both up and down. However, even in this situation, the abrupt short-term change in altitude may have only been about 20 or 30 feet, not hundreds of feet. Everyone was a bit green after that one.
Another form of turbulence is through an encounter with convective weather, such as a thunderstorm. This can be really exciting – in the wrong sense – and the only way to not get shaken until stirred is by avoiding them altogether. Our weather radar is our biggest ally in this regard, but it has its limitations.
Primarily, weather radar only detects moisture, specifically water droplets. In sufficient concentrations, this shows up as red on our radar display, and sometimes a bit of purple as our Doppler radar detects turbulence in the form of lateral movements of particles.
Thus, dry hail does not show up at all, and can really spoil one’s day. There are plenty of pictures available of aircraft that have flown through hail, and they are not pretty. This type of convective encounter is where hundreds of feet of height loss (or gain) may occur.
I suppose I should stop de-mystifying the dark art of aviation. Being seen as a wizard of the airways is key to keeping up the notion that we need to be paid handsomely to keep the lesser mortals safe.
Over and Out.