This is a possible explanation, perhaps more likely than everything discussed here so far: PREMATURE GEAR RETRACTION
This can happen due to a too early gear-up command, or because the gear lever was up on ground, with the airplane weight directly or indirectly keeping it locked down, but when the wight on wheels is reduced during rotation, the gear goes up.
This can happen due to a too early gear-up command, or because the gear lever was up on ground, with the airplane weight directly or indirectly keeping it locked down, but when the wight on wheels is reduced during rotation, the gear goes up.
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