Just a few days ago, we witnessed the tragic crash of Jeju Air flight 7C2216. We don’t want to speculate at all on what led to this, but videos show the aircraft touching down without any of the landing gear extended and without flaps or slats, which led us to asking ‘what does an aircraft need in order to land, what sort of backups do they have and what does the loss of various systems mean for a safe landing?’ In this piece, we’ll explore the systems used in a standard landing and the redundancies in place for when things go wrong.
The landing gear
The landing gear assembly on modern aircraft consists of the main gears and the nose wheel. Depending on the size of the aircraft, there will be a varying number of wheels, and check out the likes of the ginormous A380 and you’ll see some extra main assemblies thrown in too (the A380 actually has 5 gears made up of the wing landing gears, two body and the nose because it is just so darn big).
For obvious reasons, landing without an undercarriage, often referred to as a ‘belly up’ landing is an extreme situation. The aircraft will skid on the underbelly and the engine pods, and this causes a huge amount of friction and potential fire risk, but also means there is next-to-no directional controllability or means to decelerate. In this situation, pilots would look to land at the lowest possible speed and on the longest possible runway. In the past, foam was often used to reduce the chances of fire, but modern aircraft are built from fire-resistant materials and with fuel tanks designed and positioned to reduce the likelihood of fire in these sorts of situations so this doesn’t tend to be used so much anymore.
Landing with just some of the gear not extended is still very serious, but the pilots can control it to more of an extent. Without a nose wheel, they won’t have directional control, but can control the lowering of the nose of the aircraft to minimize the impact of touching down. Likewise, without a main gear extended, the pilots can control the aircraft to touchdown on the side that is extended, and use the flight controls to maintain the wing up on the other side as long as possible, controlling the lowering of it to reduce the impact on the wing and engine pod.
In general, these would all be situations that the pilots would be aware of prior to the landing, so decisions about the best runway to use (less cross wind, a long and wide runway, and one without close in obstacles that the might impact for example) would be made. And so would briefings to ATC, the cabin crew and the passengers so everyone was prepared in advance.
What if some of the tires have failed?
This isn’t such a rare event. Loss of tire pressure and even blown tires can occur. If it is just a tire or two that is blown then the situation isn’t too bad, but the braking functionality will be reduced, and there is a chance of fire on the landing gear because of the friction (and sparks) that it will generate during the landing roll. So most likely, the aircraft would stop on the runway and require towing off after it has been checked and secured.
Most landing gears are typically held up with mechanical uplocks and extended using hydraulic pressure. But aircraft have multiple hydraulic systems, so if the ‘normal’ one for extension fails, there will be a backup system (or potentially an electric backup like you see on the Boeing 787). Failing that though, there is also the possibility of using gravity to just “drag” the system down and into place. This is done following a checklist which generally requires the aircraft to be at a certain speed and nose attitude to help assist with the extension and ensure it properly locks into place.
So, the chances of having so many failures that the gear cannot be extended at all are actually very remote.
“But what,” I hear you ask, “about the pilots forgetting to lower it?”
Well, fair question, but worry not – there are a lot of barriers in place to prevent this. First, it’s just one of the big things which pilots know they have to do, and they do it every flight, so the chances of just randomly forgetting are pretty remote. But, if they do, then there are warning systems in place. The EGPWS system will automatically call out (very loudly) “too low, gear” if the aircraft descends below a certain altitude without the gear down. If landing flaps are selected without the gear being down, they will also get warnings, and generally these are non-cancellable meaning the crew can’t just ignore them.
Then there are things like checklists to catch omitted actions. On modern Boeing and Airbus, these are electronic so they will warn the pilot if not completed or if any items on it have not been completed. So the chances of the pilots just forgetting and not realizing are, thankfully, low.
OK, so we need a gear (but can land without it). Do we need brakes?
Well, brakes are very handy. Aircraft tend to use carbon fibre disc brakes which are clamped against the wheel to slow the aircraft’s wheels. They are the primary (and most critical) method for decelerating the aircraft. Thankfully, as with other critical systems, aircraft also tend to have multiple redundancies in the brake system as well. These tend to consist of things like electrical back-ups for the actuators, hydraulic accumulators which guarantee a minimum level of brake application if all other hydraulics are lost, etc.
And just forgetting to brake isn’t really possible either. Most modern aircraft use auto-brake systems (which have built in anti-skid in case you’re wondering). When the aircraft touches down, with the auto-brake armed, it will automatically start applying the brakes to meet a set deceleration rate. This rate is checked during the landing performance checks using actual conditions (and this has a lot of margin built into it) so the pilots should select an auto-brake level that enables them to stop with a lot of room to spare.
If the auto-brake system fails, then checks during the landing roll would catch this and the pilots would manually brake. While individual brake units might fail, loss of all braking is fairly unlikely but if it does, they still have the back-up of reverse thrust and other deceleration devices to assist if things are getting tight. Without full braking – for example, if only half the required brake pressure is applied – then the Boeing 787 will take around an extra 1000 feet to stop in (about 300m).
However, in certain conditions – like on contaminated runways – the friction between the tires and the runway surface can be reduced enough to prevent good braking action (hence the importance of checking the landing performance with accurate conditions). These checks also have a section where the pilots can include any non-serviceable systems so calculations include only the systems they actually have working. Again, if they’re in a tight spot where a landing has to be made in less than ideal conditions, preparations would be made prior to this.
Brakes aren’t the only way to slow down
Many aircraft have reverse thrust – this works by deflecting the exhaust air or air flow from the engine by opening up deflector doors. Even at idle thrust, there is still significant forward thrust, so the system works by changing the direction of the air flow, usually to around 135 degrees (sending it fully forward isn’t aerodynamically possible), and using that to add to deceleration.
Many commercial aircraft use a clam shell or a target type system. This sort of system opens up a section of the outer engine shell and shifts it behind the engine (or into the air flow) to form a sort of bucket. The air flow hitting this creates a lot of drag. The cold stream – the other common system seen on commercial aircraft – has doors set in the bypass duct which open and redirect a portion of the aircraft from the fan section. Both work off good old Newton’s law “for every action there is an equal and opposite reaction”…
The system is actuated by the pilots after touchdown. With the thrust levers back at idle and other conditions met (things like weight on wheel sensors to ensure the aircraft is definitely on the ground) a lever is unlocked allowing the pilots to pull it up and this sends a signal for the reversers to deploy. They are normally hydraulically or electrically actuated.
Reverse thrust does help decelerate the aircraft quickly, but is primarily there to reduce brake wear. On aircraft with auto-brake because this targets a set rate of deceleration, this rate would not be increased by the use of reversers, it would just reduce the amount of brake application pressure required to meet it. Often, only idle reverse is used, because it reduces engine wear and is also a lot less noisy. When it really has a lot of benefit is on contaminated runways, where brakes become less efficient because of the reduced friction.
Aircraft also have the speed brake system. This uses spoilers on the upper wing surface which deflect upwards. In the air, these help slow the aircraft down, and on the ground they increase drag and decrease lift on the wings, helping “put weight” onto the wheels for better ground contact (and braking efficiency). These are automatically deployed when conditions on landing are met, but there is a lever in the flight deck which the pilots “arm” to ensure it all automatically pops up on landing. They can also manually do it if it doesn’t work.
Without the speed brake, the landing distance of a Boeing 787 can be increased by around 1000 feet (300m), but without speed brake and without reverse thrust, it is going to need around an extra 1500 to 2000 feet (approximately 500m), which is a lot! So both are extremely useful systems, but are ones the aircraft can land (and stop) without should they fail.
But what else does an aircraft actually need in order to land?
Well, flaps and slats are important because they enable the aircraft to reduce to a much lower approach speed. Lower speeds means shorter stopping distance, and also a lower nose attitude – that’s helpful because it means the pilots can actually see outside. But without these, the aircraft can still land, it is just a lot more difficult and takes up more of the runway.
A major system of critical importance though is the hydraulics. These tend to actuate flight controls, including flaps and slats, but also ones for directional control. But, even with the loss of all hydraulic systems, aircraft will still have some level of backup. The very last level of support comes from a system called the RAT – the Ram Air Turbine. This looks like a little propeller which drops down from underneath the aircraft automatically in the event of certain losses (and can be dropped manually by pressing a switch in the flight deck as well).
It is fairly basic, but can provide enough hydraulic (or electric) power that the absolutely necessary systems are still available and functioning, and even without that, aircraft will still fly. Back in 1989, a United Airlines DC-10 was forced to land at Sioux City after a catastrophic engine failure resulted in the loss of all their hydraulic systems. The aircraft was controlled using differential thrust on the two remaining engines and while it did break apart on landing, there were survivors and it came down to the exceptional skills of the pilots.
What about engines?
Well, again very handy. Without these the aircraft is going to basically become a large and very heavy glider. It will also lose a lot of the systems which run from the engines – namely electrics and hydraulics (save for any emergency back-ups which the APU or RAT provides).
A famous example of this is the Gimli Glider accident of 1983. This Air Canada 767 lost both engines when they were starved of fuel. This led to the loss of all engine driven hydraulic and electric systems, and they just had their RAT to rely on. The crew managed to glide to an old air force base and touched down on the runway. While the nose gear collapsed, there were no fatalities.
Electrics?
Well, the Boeing 787 is arguably the most electric driven aircraft, but the level of backups and redundancies onboard mean the loss of absolutely everything is incredibly unlikely, and again, systems like the RAT mean a basic level of functionality is always provided.
Emergency power level – when you only have the battery left – drops the aircraft down to a very basic level where the pilots would only have basic flight controls and instruments available to them. It would be dark, challenging and not very fun… but there is no reason the aircraft could not be safely landed.
Safety through redundancy
Aircraft can land with hugely reduced systems, and they have so many levels of redundancy, that reaching these degraded levels is extremely unlikely. Enough systems will still run with only a battery providing electrical power that the pilots will be able to reach an airport and get safely down. An aircraft can still be controlled even with the loss of all the main hydraulic systems, and it can still land (and glide a reasonable distance) even without its engines running. While the landing gear is critical, successful landings without it have been made.
But as with everything, ‘success’ in these situations comes down to a lot of other factors, and often conditions can make this more challenging, and human factors will come into play. The accident at Muan airport is an incredibly tragic one, and until an investigation has been concluded, we won’t be able to understand how precisely how this occurred or what the industry can learn from it in order to prevent it from happening again. What we can say is it is an extreme one which, while unlikely to occur again, will likely lead to new safety measures for the industry.
