ancman

My guess is that it’s a holdover from earlier days when STAR and ATC-assigned speeds were less common at 10,000. It provided a buffer below the 250 KIAS speed limit. I agree that it’s still a stupid default.

OOfff

but why do we need a buffer? An accidental excursion to 254kias because of a gust doesn’t matter and won’t get anyone violated. You’re probably right that it’s a buffer, but it’s a stupid buffer

2StgTurbine

If Boeing thought we needed a buffer in the descent, why not also on the climb?

m3113n1a1

My guess is it's a buffer for flap speed. So you don't forget to slow down before you go to flaps 1 or 5. Still dumb though.

beernutt

The Corleone family had a lot of buffers.

- Willi Cicci

TED74

VNAV protecting the descent page speed (default 240) will allow it to be exceeded by up to 10 knots. If you change it to 250 (to avoid “unable 250 at XYZZZ”), you accept the jet being 10 fast (260) when it resumes descending below 10k after autonomous slowing. Made sense to an engineer, I suppose.

Gone Flying

I thought so too when I flew the 737 but the 757 has the same limit and flaps 1 speed for the -200 is 240 knots

saturn

Correct. Just funny, because we all know how uncomfortable the PM would feel if below 10k feet we bugged 255 kts. But setting 205kts when assigned 210, the PM: "I like it. Savvy move". And of course, if ATC asks "say speed" when you're +/- 10kts.. the only correct answer is always..whatever they assigned you. I see the whole 10ft of altitude the same way. I think we can all see the big picture here.
No, the lazy PF [and maybe wiser] just hits DES right away, and lets it go 1,000fpm until capture. Yes, Auto descent on the bus takes more work, so it makes no sense. For that reason, if somebody is doing it, it's the opposite of lazy. More like bored/curious. It's not prohibited, dangerous, ot reckless. Just inefficienct. That's why it's not encouraged.

Kinds like if "descend via arrival" other guy wants to do it all in V/S or FPA, or by hand.. whelp, ok. Probably bored AF. As long as both pilots are up to it, who cares. Doesn't mean it's a good idea all the time, or you'd chose to do it on a line check or LOE. But it's OK.

If you build a PBD over the orginal TOD, wouldn't the geomtic and idle path be the same? Why would the engines not be at idle the whole way down? Same distance to lose same altitude.
And like above, you'll never hear a peep about a pilot who hits managed descent early on a PD and burns up more gas. Cuz he's managing a threat.

Meme In Command

Yeah technically you're right you run the risk of ATC assigning a speed for sequencing and now your PBD may not be good to meet a crossing restriction.

To your point about starting early, 10ish miles prior 99% of people won't say anything. Anything past 20ish miles prior and most captains I've flown with will say "eh, let's wait a little longer, don't you think?", above the checklist in the glare shield and watch the arrow.

OOfff

which is funny because the fuel savings from waiting are immeasurable. Who cares if we start down 20 miles early?