When pilots begin to slow the airplane for landing, the first big step is to lower the landing gear. In jets flying an instrument approach, the wheels are normally let down at the final approach fix, already five to six miles from the threshold, so the gear is locked well before the runway appears through the clouds. On a sunny day jet crews use the same five-mile point, guaranteeing the aircraft is in a stabilized, configured state with drag, trim and checklists complete.
For light general-aviation airplanes the cue is visual: abeam the touchdown point on the downwind leg the pilot selects gear down, then adds flaps while turning base, keeping the sequence well above the 500 ft - 1,000 ft gate. Extending the gear only below the published maximum speed protects the doors and linkages from damage and, once the three green lights show, the aeroplane is deemed ready to land. If the landing gear fails, the crew must prepare for a gear-up, or belly-landing instead.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
When does the landing gear go down?
The landing gear is lowered during the final approach, most commonly when the aircraft is six to five miles from the runway. In reduced visibility it is extended at the final approach fix, approximately five miles out. Company stabilized-approach guidelines require the gear to be down by 500 ft (152.4 m) in visual conditions and 1,000 ft (304.8 m) in instrument conditions, always below the maximum extension speed. Pilots therefore select gear down on the cockpit lever once the airplane is slow enough - typically below 280 kt - because above that speed the system will not extend the gear until speed has decreased.
During a visual circuit in light aircraft the sequence is slightly different: the gear is dropped abeam the touchdown point on the downwind leg. On the base leg the extension gives useful drag and extra time to slow down. The lights on the panel illuminate green only when each wheel is down and locked at which point the pilot double-checks that the gear is safely in position.
Does the gear go down automatically? In normal flight, it does not. Extension and retraction are controlled manually by the pilot monitoring through the selector lever, although the aircraft's logic will inhibit extension above 280 kt and some types feature auto-gear lockouts at high speed. Once the command is given, hydraulic or electric actuators drive the gear to the locked-down position in about seven seconds, and gravity assists if the alternate free-fall system is used.
Altitude has little direct effect on extension timing, but the gear must be below 15,000 ft (4 572 m) if intentionally left extended for cooling. The decision when to drop the gear ultimately rests with the captain, who balances the need for a stabilized approach, adequate drag, cooling brake temperatures, and company policy.
What happens if a plane's landing gear does not come down?
If the gear fails to extend, the crew first tries the normal lever, then the alternate extension mechanism. If that also fails there is no way to access the gear to unjam it and the aircraft must perform a gear-up or belly landing, touching down without some or all of the landing gear extended. A belly landing means the fuselage skids on the runway. The impact forces are much greater because no struts absorb the shock, so the aircraft receives substantial damage, although injuries to occupants are often minor or none.
The landing gear could be stuck up, stuck down, or partially retracted. A down-lock hook or uplock jam, a missed step in removing a pin, or a mechanical error in the hydraulic system prevents movement, hydraulic fluid loss, contaminated fluid, or a power-pack fault freezes the gear. Sensor disagreement, contradictory gear-status lights, or an EICAS/ECAM gear-fault message will alert the crew.
What if the gear does not come down and fails every extension option? The crew follows the QRH abnormal checklist, briefs the passengers, disables anti-skid, dumps fuel to reduce weight, and requests emergency services on landing. Approach is flown at minimum flap setting and touchdown is kept nose-high as long as possible to spare the fuselage. Skidding to a quick stop on the belly is safer than overrunning the runway on wheels.
What if the gear fails to retract after take-off? Landing gear not retracting is relatively rare but causes a massive increase in fuel consumption and limits maximum speed because open gear doors are torn off by the slipstream. The pilot has three options: continue with the flight, return to the airport of origin, or divert to another airport with full gear capability. The decision is made after discussing the problem with maintenance control. Once on the ground the pin is removed or the hydraulic fault is corrected, and retraction is attempted again.
Can a plane fly with landing gear down?
Flying with the gear down is still safe, yet it is far from efficient because the extended undercarriage adds drag and makes the airplane less aerodynamic. All aircraft have a maximum speed limit - called maximum landing extended speed, or VLE - at which the airplane can be flown with the landing gear extended. Flight above that speed risks structural damage. Flight with gear down involves limitations on cruising altitude and indicated airspeed, and the resulting drag shortens range so the airplane will not fly as far as planned. Pilots therefore treat gear-down flight as a short-term measure. On approach they leave the gear lowered for several minutes, but for the rest of the flight standard procedure retracts the gear after takeoff once a positive rate of climb is verified. Although an aircraft can land and even take off with gear extended when a technical precaution or failure exists, such operation is not standard procedure.
