Boeing 787's Unique Landing Gear System: Early Doors Function
The Boeing 787's landing gear retraction system is unique among commercial aircraft. Unlike most planes, where the main landing gear doors open only after the pilots confirm a positive rate of climb and move the gear lever to UP, the 787-9 and 787-10 Dreamliner models have an 'early doors function' that begins opening the doors automatically shortly after take-off.
This feature is triggered by the aircraft's air-ground sensing system about one second after take-off, when it recognizes that the main landing gear struts have extended. At this point, the gear itself remains down, but the main landing gear body doors start their opening sequence automatically. The pilots can leave the gear lever in the DOWN position while the doors open, as the aircraft knows it is airborne and doesn't interpret this condition as a command to retract the landing gear.
The system's importance becomes clear when an engine fails immediately after take-off. In such cases, climb performance is crucial, and every increment of lift matters. By opening the doors early, Boeing has created a system that allows the aircraft to start retraction sooner once the pilots give the command to retract the gear.
The reason for this design choice becomes apparent when considering the conventional retraction sequence. Landing gear doors do not move instantaneously, taking approximately three seconds to open from closed. Meanwhile, crews conventionally initiate gear retraction after confirming a positive rate of climb, around three seconds after take-off. With a conventional sequence, roughly six seconds or more can pass before the landing gear has even begun moving toward the fuselage.
The Boeing 787's early doors function effectively starts part of the retraction process ahead of the crew's gear-up command, allowing the subsequent movement of the gear to begin sooner once that command arrives. This results in a shorter period in which the aircraft carries the full aerodynamic penalty of its landing gear during the initial climb, translating into a small but useful performance advantage.