PPOS hold on B-738-LNAV,VNAV..

[boeingdream787]

Just something that happened to me today.Say you get into a PPOS hold,in straight and level flight at say 50 nm from destination.Both LNAV and VNAV are the operational modes.Say you are descended to an interim level from your cruising level by ATC and asked to hold Present Position(PPOS) at say level 250 and your cruising level was 350.My question is that once in the hold,without ANY pilot input into the FMC's what would the FMC/AP do and why.In this instance,say you have a reserve fuel of 2.0 T and you had 2.5 T when you started the hold.No EAT fed into the box(would that make a difference??).Would REALLY help if i could get something from ALL the learned ones out there.Many many thanks in advance.BTW....737-800.[/COLOR]......:confused:

[blackmail]

boeingdream,
the aircraft would remain in the holding pattern until running out of fuel. it happened a few years ago with the helios 737 accident over athens(greece), where the cockpit crew went unconscious due to lack of oxygen because of confusing in mishandling the pressurization system. the plane continued on autopilot & went into a preprogrammed holding until running on empty. the fmc would just put a msg in the scratchpath:" using reserve fuel".
kind regards,
bm

[boeingdream787]

Blackmail,thanks.But i was thinking differently.IF there were no discontinuties in the FMC,and if this were a PPOS hold(not one over destination,like helios,OR one enroute destination,but PPOS),i would think that there must HAVE to be some inbuilt protection.After all the airplane is still flying VNAV speed and there has to be a protection against a stall in VNAV speed(obviously this would'nt work if the engines have flamed out).But there MUST be SOME protection.......:confused: .Welcome any input.Thx.

[Kit d'Rection KG]

No, there's not.

I might say, why should there be?

There are two paid professionals in the flight deck - unless you're hinting at the Helios accident - and for them, LNAV, VNAV, and all the other toys, are just tools...

[blackmail]

hello boeingdream,
if you fly lnav/vnav & for some reason (atc, weather, operational, whatever... .), you want to hold at present position, on the cdu, you select "hold" mode, then line select the "ppos" prompt(> or caret as it is called), execute & the aircraft will start a righthand turn(standard holding pattern), reduce to the holding best speed as calculated by the fmc( circa 210-220kts = min drag speed) and do this until the fuel is gone. it will give you the hold time available as an advisory, by calculating the fuel backwards : reserve fuel from map + fuel required to fly from ppos hold to map via the programmed route.
you have indeed speed protection modes when flying with all the automatics, AFDS(Autopilot Flight Director System) & A/T(autothrottle) engaged. the aircraft will not fly too fast nor too slow & will respect flap & gear placard speeds. in some flight regimes, speed & A/T modes will not be sufficient & a "performance reversion mode" will automatically activate. e.g. if a high V/S(vertical speed mode) cannot be sustained in climb/descent, without a speed reduction below/acceleration above a minimum/maximum speed, the the system will revert to level change, the nose will pitch down/up as to keep the speed just above the limits. it will also work in vnav path descent(reversion to level change descent), but it(the reversion mode) will not work in our example when flying level in vnav. normally the A/T will keep the FMC calculated speed, but, if for some reason(failure or pilot intervention), the A/T reduces thrust & disconnects, you will have a speed reduction & stall.
last point, the automatics are a terrific tool & help, if well understood, but the thinking & management must be done by the crew. fmc & autopilots are just sofisticated computors obeying to sod's law: "garbage in/garbage out"
hope this helps.
bm

[boeingdream787]

Blackmail........as usual.You've come out up and ahead!! Many thanks.This is very very close to what i had originally thought.But i still feel that there would be SOME kind of a "reversion mode"even in level flight VNAV path.Dunno why but its still bothering me.mebbe i gotta dig deeper.Many thanks in the mean while Blackmail....( second time in the past week......lolol)Cheers now.

[blackmail]

boeing dream,

in level flight, A/T provides speed protection & speed control. reversion mode is not available because, if required, that would mean a departure in level change from the mcp selected altitude or vnav altitude restriction, something mr.boeing doesn' t want to happen. e.g. how many times( in initial training & later) did we try to select V/S to no avail because we forgot to FIRST select a new altitude on the mcp?
many thanks & always welcome,
kind regards,
bm