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I know that landing gears can be deployed even without hydraulic power by letting them go down by gravity, as seen on this video:

I was wondering: do the landing doors need replacement after the landing? It looks like the get damaged a lot, especially on hard touch downs.

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No, they do not get damaged.

The Main Landing Gear (MLG) or Gear Bay doors can and are regularly opened on ground for maintenance purposes. Even with fully compressed oleos the MLG doors are still off the runway surface. Have a look at this Air Canada A320. You can clearly see the doors being well above the bottom of the MLG.

If you came to your conclusion by watching the white puff of smoke coming from the area of the open MLG doors - that's actually the burning rubber from the nose gear touching down.

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  • $\begingroup$ It really depends on the aircraft type. There are some aircraft that will drag the doors on the ground if there is no hydraulic pressure. (BAC 1-11, DC-9, Fokker F-28, etc) $\endgroup$
    – Mike Sowsun
    Sep 9, 2019 at 14:17
  • $\begingroup$ uhm, looks like there is little clearance between the landing doors and the ground, a wheel radius worth of space. At least on a A320 -> youtube.com/watch?v=g97WyYvKLfQ $\endgroup$
    – Gianluca Ghettini
    Sep 9, 2019 at 14:25
  • $\begingroup$ The problem with non-hydraulic deployment is that many planes will not positively lock the gear down, so it becomes a guessing game whether the gear will collapse when the runway is reached. In a TV series called Ice Pilots NWT, flying out of Yellowknife, North West Territories in Canada, they had a Lockheed L188 Electra where the gear got stuck on the doors on one side. Pilot tried landing on the good gear twice hoping to shake it loose with no luck. Ended up landing on one main and nose gear, and tore the low wing props all up as it came down the runway, finally stopping with no injuries. $\endgroup$
    – CrossRoads
    Sep 9, 2019 at 18:16
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    $\begingroup$ @CrossRoads, the L-188 is a design from the mid-1950s. Design standards have changed a bit since then. $\endgroup$
    – Mark
    Sep 9, 2019 at 22:40
  • $\begingroup$ My initial reaction was that there was just tire smoke. However, it appears that there may be some sparks shortly after MLG touchdown at about 0:27/0:28. Upon further watching, that may be smoke from the NLG, though. $\endgroup$
    – FreeMan
    Sep 10, 2019 at 18:51

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