when we land we need to make sure we have enough Runway to stop our aircraft safely otherwise we might send our big heavy aircraft flying off the end of the runway into a field into the sea or maybe even into some buildings how do we figure out how much distance we need though let's find out [Music] hi I'm Grant and welcome to the 11th class in the performance Series today we're going to be taking a look at the Landing phase of flight this is going to be the final video covering all the different phases of flight before we move on to class A regulations class b regulations and looking at some graphs in the cap 698 document but before we do that we're going to have to learn how to land and more importantly stop an aircraft safely The Landing distance required for an aircraft starts from the screen height until the aircraft comes to a stop on the runway this means that there is a section of Landing distance which is actually in the air and also a ground run section and the screen height that we're talking about is usually 50 feet for a class A and 35 for a class B aircraft the screen is an imaginary wall that is placed on the runway threshold that the lowest part of the aircraft must clear in order to land safely and remember that thresholds can sometimes be displaced so it's not necessarily the end of the runway it could be say the displace threshold was here you'd have to cover this bit by 50 feet before landing calculations are done and tested by manufacturers and displayed in graphs are basically done according to passing this screen height at the correct altitude and at the correct speed and say for instance we were too high then we touch down at a later point on the runway in the brakes we'd only be able to be applied later and we therefore need more distance to land so it's important that we cross the Threshold at the screen height and at the correct speed the speed that we aim for in landing is a speed called V ref basically the reference speed for landing this is the speed that we cross Threshold at and the screen height at and vref is calculated by comparing the minimum control speed for landing which is vmcl [Music] um and the stall speed in the landing configuration which is the s0 and we apply a safety factor to the stall speed so we do actually reach the stall which is 1.3 so it's this versus this and vmcl if you're not familiar is basically a safe enough speed to fly at so that if we go around there's enough flow over the aerodynamic surfaces to maintain control of the aircraft and the stall speed in the landing configuration is obviously going to keep us above the stall speed in the landing configuration and we Chuck on 1.3 as a safety Factor just to make sure we're flying fast enough so if we had an aircraft with a vso of a hundred knots and a vmcl of 140 knots then we multiply that well 1.3 we get 130 knots and we use the higher of the two speeds so our vref in this occasion would be 140 knots simple as that if we were to fly faster than this speed then we'd have to slow down from a faster speed increasing the total Landing distance required and if we flew any slower we'd beat risk of stalling or being unable to control the aircraft in case of a go around so it's important to fly as close to vref as possible when passing over the threshold of the runway after we have initially touched down we can start to do things to slow us down the most effective thing to do is to use the brakes brakes convert the kinetic energy of the aircraft into heat energy in the brake pads themselves and in the wheels to slow down the plane down and therefore they can get quite hot if it's a particularly hot day you might come close to the high end of the safe range of temperatures to operate the brakes in and you might need to say you're turning the aircraft around you're Landing somewhere and taking off soon after you might actually need to wait a bit for the brakes to cool down into a safe usable range again in order to get the most effective deceleration from the brakes we need to make sure that the whole weight of the plane is on the wheels as opposed to some of the weight being counteracted by any residual lift that might be passing over the wings and being generated by air passing over the wings this is done in large Jets by using ground spoilers basically little flaps that pop up off the top of the wing you've probably seen them when you've been Landing looking out the window on a plane and they spoil ground spoilers or ruin the lift generation of the Wings and that dumps the whole weight of the aircraft fully onto the wheels and it's especially important that we get the full weight of the aircraft onto the wheels quickly when the runway is wet for example or it's got ice or snow in it and this is why you'll sometimes get quite heavy Landings if the conditions are wet and horrible it's because the brakes sorry the weight is on the wheels quicker and the brakes can be applied sooner under these slippery conditions at least that's the excuse I use whenever I have a heavy Landing so there are a few things common on Commercial Transport Aircraft that also help with braking the first of these is anti-skid anti-skid applies the brakes on and off very very quickly so that the wheel doesn't skid the clues in the name it's anti-skid this is because a break only works whilst the wheel is spinning we're trying to slow down the rotation with the brakes not entirely stop it that's because if the wheel completely stops rotating then the tires will slide on the surface and we have no control over the speed whereas if they're continuously rotating then we can slowly reduce the speed of the wheel rotation all the way down to a safe taxi speed anti-skids basically detects the first signs of Tire skidding releases the brakes allowing the wheel to rotate again before applying the brakes again doing this many many times per second and anti-skate is particularly useful in wet and contaminated conditions because the wet surface reduces the grip on the tarmac and makes the torque the tire more likely to Skid in the first place so if we have anti-skid we're less likely to Skid on a wet or icy Runway a second helpful tool on most Airlines is an auto brake system again the Clue's in the name it's an automatic braking system and this system basically applies the brakes very shortly after touchdown meaning that an element of crew reaction speed is taken out of the equation and the aircraft slows down regardless meaning it will stop in the available distance the second most important thing used to slow us down in a landing situation is reverse thrust this basically Works in a propeller by finding off the pitch of the pillars until we get a negative angle of attack at the propeller blades ending up in a resultant force that is backwards instead of the normal forwards Direction and that is going to slow us down that is our reverse thrust in a jet we essentially redirect the flow of air so instead of the air flowing through the engine this way creating a resultant Force forwards we redirect some of that air so it's now flowing in and back forwards and that creates a resultant Force which is backwards that is what we feel as reverse thrust and this is something you can see online and there's a few different ways to do it but you sometimes see it as just little doors opening on the side of the engines or sometimes it's like a whole part of the engine cowl slides back and when you hear like really loud noises if you're near an airport um just after an aircraft's landed it's usually this because the air is now hitting a surface rather than just like flowing through the engine if that makes any sense so reverse thrust alone probably won't be enough to stop an aircraft so they must be used to assist with braking therefore the total decelerating force is a combination of breaks and reverse thrust and to some extent aerodynamic drag from things such as the spoilers popping up and creating a bit of excess drag so say for example we used a lot of reverts thrust then we wouldn't need as much braking Force to make the total decelerating force that we require and the reverse would be true as well so different airlines will have different policies on what combinations are used as there's a bit of a trade-off so if we used a lot of reverse thrust then the brakes won't get used as hard and they won't get as hot so we would be able to turn the aircraft around quite quickly and we wouldn't get as much wear on the brakes in general the contrast of this is that we're going to be working the engine harder meaning there's more wear on the engine and potentially higher maintenance cost so it's basically do you want to spend more money maintaining the engine or do you want to spend more money maintaining the brakes and generally speaking brakes are going to be cheaper to replace so we use more brakes and less reverse thrust and reverse thrust is also quite noisy so at night time you might use just very very low levels of reverse thrust and high amounts of braking so there's various things that will influence our Landing distance requirements so if we are heavier then we need to fly faster in order to achieve the correct amount of lift our stall speed increases in other words so that means that our uh speed for vref is likely to be higher as well because we're taking the lower of the V sorry the higher of the stall speed times 1.3 or the vmcl value and that faster V ref speed results in a faster touchdown speed and they're more therefore more distance is required in order to slow us down to a stop so as mass goes up the landing distance requirement also goes up so altitude and temperature are both sort of similar because they increase the sorry as they increase the density of the air decreases and one way to think of it is using the equation for Lift again so say we have a lower density we need to fly faster same story as over here we're going to be flying a faster stall speed faster uh bmcl speed taking a higher V ref speed Landing faster more time to slow down and the other way to think about it is basically that if it's less dense air it's a faster true air speed for the same indicator speed and I would default using these three fingers again and say we're increasing in altitude our Landing V reference speed is 140 knots from before but if we're increasing altitude that means our true air speed is actually going to increase a bit as well so if we're higher up we're going to be landing a faster trigger speed which means touching down at a faster speed taking more distance to slow down and temperature you know the temperature uh decreases as allergic increase Etc so if we have a strong headwind on Landing then we don't need as much distance to stop this is because our ground speed will be a lot slower say RV ref for example was 140 knots and we have a 20 knot headwind then that means that our indicator speed of 140 knots is a ground speed of 120 knots plus the 20 knot headwind that we're feeling over the wing and providing the lift and the speed indications that we're reading this means that on Landing instead of slowing down from 140 knots we're actually only slowing down from a ground speed of 120 knots this means less distance required to slow down to a safe speed Tailwind would be the opposite case and as wind is variable and unreliable we only use 50 of the headwind and we use 150 of the Tailwind components so that we're not overly relying on this inconsistent wind so braking depends on the wheels gripping the runway surface if they have nothing to grip onto then they might slide and the brakes don't work if the wheels are not rotating so the best conditions for grip are when it's dry and when the rum May is made out of nice friction filled tarmac if anything deviates from this then the brakes or more specifically the tires won't be able to grip and slow down the plane as quickly and as fast if the runway is wet for example the normal factor to apply to your calculated Landing distance is 1.15 so say we had a landing distance required when dry of a thousand meters then our wet Landing distance required would be 1 150 meters simple and if that was going to make us go off the end of the runway or something we might need to use more reverse thrust or a higher level of Auto breathing or indeed land somewhere else so the slope of the runway is also important so if we land into an up sloping Runway then some of the weight of the aircraft pulls us down the slope and helps with slowing down the aircraft and if we were to land down the slope then the weight of the aircraft would pull us down the slope and make it harder for us to stop the effect is usually quite small though because runways are pretty flat generally speaking and it's not like we're going to be landing into a 45 degree slope like this so the effect is pretty minor so because humans fly planes it means we don't always cross the screen higher exactly 50 feet at vref and don't always apply the brakes or select reverse thrust at the ideal time we might also make slight variations to The Landing and flare technique in general which means that two identical aircraft on the same day flown by different people might have vastly different Landing distances we therefore apply a safety factor to cover for these variations we essentially don't want to use all of the runway we want to have some left over to account for these changes for a jet for example we want to land in the first 60 of the runway and for propeller it's 70 percent this means that when we calculate our Landing distance required for a jet we need to multiply it by 1.67 and that needs to be less than or equal to the total Landing distance available and for a propeller it's the landing distance required multiplied by 1.43 and that needs to be less than or equal to The Landing distance available or alternatively you can take the total Landing distance available and multiply that by 70 percent or um 60 over here and that has to be that's your new factorized Landing distance available and your Landing distance required would not be allowed to be over that distance