Joe was getting on the famed aircraft Tuesday as he was getting ready to leave D.C. for California -- where he'll be in town for a few days as he fundraises for his reelection campaign ... but as we've seen before, he had somewhat of a hard time climbing aboard.

Check it out ... you can see JB make his way up a shorter staircase into a different part of the plane -- which is much lower to the ground than the usual entrance he might take -- but even here, the Prez struggles a bit ... jolting forward a couple times as he catches himself.

Luckily, he didn't fall here -- as he has in the past -- but it was a little jarring. Even more surprising is the fact that a Secret Service agent was stationed at the bottom of the stairs ... keeping a watchful eye on his boss. We've been seeing this more lately with Joe.

It's interesting ... the agent at the bottom of the (shorter) stairs is something the NYT reported on earlier this month -- part of a new strategy in managing/shielding Biden.

Worth noting ... Donald Trump would also use the shorter stairs at times during bad weather, but now -- it seems Joe taking the shorter stairs with someone monitoring him from behind is the new norm. Even here though -- it seems like a potential fall is still in the mix.

It goes without saying ... these concerns have been mounting -- namely, ones about his age/fitness for office. It's something he's swatted away time and again ... but folks are talkin'.

Also worth noting ... this ain't the first time Joe has fallen during his presidency. He's taken some nasty tumbles before -- including while boarding Air Force One.