You only need to look at the problems Boeing have had with the 737 models to realize this. However, there is only so much stretching you can do without needing to completely redesign the aircraft. A great example of this is when Boeing took the original 787-8 and stretched it by 20 feet to create the 787-9. Manufacturers can stretch an original aircraft design and create a load more space for passengers and cargo and this can often be done without a complete redesign. This results in pressure coming off the switches, letting the aircraft systems know that the aircraft is now in the air. When the aircraft gets airborne, there is no longer weight on the wheels. When there is "weight on wheels," these switches are compressed telling the aircraft that the aircraft is on the ground. On each of the main landing gear beams, there are micro-switch sensors. So how does the aircraft know where it is? As you'd hope, the landing gear lever will not allow the gear to move to the up position when the aircraft is on the ground. Read more: Cabin pressure: How pilots avoid disaster in the cockpit Air-ground sensing systemĬertain systems only work on the ground, and others in the air. This makes for a much more straightforward and easier maneuver to fly. On the -9 and -10, the early gear door opening eradicates this problem as the initial pitch angle we must fly does not change. If not done carefully, it can result in the aircraft "porpoising" as we attempt to fly the ideal flight path. With this adjustment made and the landing gear retracting, we then have to raise the nose again. As the gear doors open, the change in aerodynamics causes the TOGA reference line to dip, requiring us to lower the nose slightly. On the 787-8, at the moment, we've just found the right nose-up angle, it's time to bring the gear up. To hit the sweet spot in between these two extremes, pilots must rotate the nose up to a line projected in the head-up display called the "TOGA reference line." Once there, and the aircraft is stable, we can then transfer back to normal guidance. Rotate too quickly and the aircraft may be flying too slowly to climb safely away from the ground. Rotate too slowly and the aircraft may not get airborne before the end of the runway. If this happens, how the pilots rotate the nose into the air is absolutely crucial. This action may seem random but it is done quite deliberately.Īll takeoff performance is done presuming that an engine will fail at the critical moment of takeoff. This happens even before the pilots select the landing gear lever to the up position. To improve aircraft performance on the 787-9 and 787-10, one second after the aircraft detects that it is airborne, the main landing gear doors automatically open. When all the gear is down and locked, the gear position indicator in the flight deck shows "down" in green, as seen in the photo above. When in the down position, it locks into position to stop them from folding on touchdown. When the gear lever is moved to the down position, the gear bay doors open and the wheels free-fall out of their stowage without the use of the hydraulics system. To lower the gear, pilots simply do the opposite. As a result, before the wheels are folded away, the brakes on the main wheels are applied to stop them from spinning. Bringing them up inside the aircraft at this speed could cause some serious vibration to be felt in the passenger cabin. On liftoff, the tires will be spinning at around 180 mph. However, before this occurs, one other important thing must happen first. Firstly, the gear bay doors start to open, creating space for the wheels to be retracted into the belly and nose of the aircraft. To raise the gear, we simply move the gear lever to the up position and this starts the gear retraction sequence. The nose gear has two wheels, neither of which have a brake. Each main gear set up has four wheels, each of which has an electronic brake. The landing gear system on the 787 Dreamliner consists of two main landing gear assemblies and one nose gear assembly. In the video below (starting at 1:29) you can see how the aircraft "sits down" onto the oleos as it settles onto the runway. This uses a mix of compressed air and hydraulic fluid to dissipate the forces experienced on landing across the airframe and also to dampen any recoil to reduce the chances of the aircraft bouncing back up into the air. To absorb the shock of the landing impact, the landing gear has an oleo strut, which acts as a type of suspension. Here, manufacturers drop the gear to simulate forces experienced in not only normal and testing conditions, but also those well beyond what the aircraft might ever expect to encounter. In order to do this, before an aircraft is certified for flight, the gear must undergo a test known as the drop test.
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