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arthurfabre

Using Alt Hold

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Hi!I'm having some problems using the Alt Hold function. Say I'm preforming a vfr flight and I'm using Alt Hold to maintain FL140. Then I want to descend to 6000 feet. If I just dial in the altitude nothing happens, and Alt Intv doesn't do anything either.On the other hand, if I'm in speed mode, the plane tries to maintain every value I dial in, which seems normal. Shouldn't it be doing this with Alt Hold too?Thanks a lot!Arthur


Arthur Fabre

UAL1112 - ualva.org

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You have to use level change.


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Does not work this way. ALT INTV is an advanced function associated with VNAV. ALT INTV will get you "unstuck" from VNAV ALT, delete the next altitude constraint in your flightplan (unless a constraint is associated with an approach waypoint) or change your flightplan cruise altitude UP or induce DES NOW from cruise (in that order).In simple ALt HOLD the procedure is : change the MCP altitude. Select a vertical mode like VS or LVL CHG and then adjust vertical speed (if in VS) or IAS/Mach target (in LVL CHG).


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Does not work this way. ALT INTV is an advanced function associated with VNAV. ALT INTV will get you "unstuck" from VNAV ALT, delete the next altitude constraint in your flightplan (unless a constraint is associated with an approach waypoint) or change your flightplan cruise altitude UP or induce DES NOW from cruise (in that order).In simple ALt HOLD the procedure is : change the MCP altitude. Select a vertical mode like VS or LVL CHG and then adjust vertical speed (if in VS) or IAS/Mach target (in LVL CHG).
When I select a new altitude ( while in ALT HOLD ) the light in the switch does not come on.It works on the Honeywell no problem.Fred.

Frederic Steiner.

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It should be on if you are all ready in ALT HOLD prior to selecting a new ALT. Once you set a vertical mode, with a new ALT set, the plane will depart ALT HOLD.
I don't think you understand what I am saying !Eg: I am at 12,000 ft and flying straight and level.The ALT HOLD light is extinguished and the FMA annunciates ALT HOLD because I am in ALT HOLD.If I now select a new altitude in the MCP the ALT HOLD light should come on but it does not.Fred.

Frederic Steiner.

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Thanks for all the answers!I've tried looking this up in the FCOM but still don't get it.

Vertical Speed (V/S)Climb/DescentALTITUDE selector..................................................Set desired altitudeNote: If a new MCP altitude is selected while in ALT ACQ, the AFDS engages in V/S and the existing vertical speed is maintained.V/Sthumbwheel..............................................Set desired vertical speed
So if I dial in a new altitude, and then switch to VS mode will the plane level off at the altitude assuming VS is set in the right direction?The section on ALT HOLD wasn't of much help either...Thanks!

Arthur Fabre

UAL1112 - ualva.org

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Thanks for all the answers!I've tried looking this up in the FCOM but still don't get it.So if I dial in a new altitude, and then switch to VS mode will the plane level off at the altitude assuming VS is set in the right direction?The section on ALT HOLD wasn't of much help either...Thanks!
Yes. VS or LVL CHG will do the work.

George Golas

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I hate gravity!

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Thanks for all the answers!I've tried looking this up in the FCOM but still don't get it.So if I dial in a new altitude, and then switch to VS mode will the plane level off at the altitude assuming VS is set in the right direction?The section on ALT HOLD wasn't of much help either...Thanks!
Rather than use V/S mode, it's much better to try using LVL CHG - the AP mode specifically designed for what you are trying to do (change flight level). This takes care of autothrottle too, and allows you set set a climb or descent airspeed (or keep it as it is).Kevin Hall

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It despends really when to use Lvl Chg. It will work fairly well for the original poster at his 14,000 feet MSL, and I find myself using it almost exclusively when approach and under 10,000 feet MSL (to ensure the plane does not brake the 250 kts speed limit.But try and do Level Change at something like FL350 at Mach .79, you will be in for a world of hurt. V/S is your best bet there (assuming VNAV is somehow not availble of course).


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Always be carefull when using V/S because it can both climb or descend away from your selected altitude,and it also offers no speed protection.Boeing only recommends using it for very small altitude changes. ( eg. 1000 feet )Fred.


Frederic Steiner.

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But try and do Level Change at something like FL350 at Mach .79, you will be in for a world of hurt. V/S is your best bet there (assuming VNAV is somehow not availble of course).
A world of hurt? Why so?
Always be carefull when using V/S because it can both climb or descend away from your selected altitude,and it also offers no speed protection.Fred.
I thought that for quite a long time, but that's just not the case. V/S reverts to Level Change for underspeed.

Matt Cee

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It despends really when to use Lvl Chg. It will work fairly well for the original poster at his 14,000 feet MSL, and I find myself using it almost exclusively when approach and under 10,000 feet MSL (to ensure the plane does not brake the 250 kts speed limit.But try and do Level Change at something like FL350 at Mach .79, you will be in for a world of hurt. V/S is your best bet there (assuming VNAV is somehow not availble of course).
What can V/S do that LVL CHG cannot? If you were cruising in VNAV at FL350/M0.79 and wanted to step climb to FL390 would you switch from VNAV to use V/S to avoid the problems you mention? Similarly a climb in LVL CHG is uneventful.For small descents LVL CHG does have a problem in that it reduces thrust too much, unlike VNAV. In that case, if not in VNAV, V/S would be preferable, but you'd have to keep a close eye on airspeed.Kevin Hall

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I thought that for quite a long time, but that's just not the case. V/S reverts to Level Change for underspeed.
Only for underspeed, and then only on the edge of stall (assuming the NGX modelling of the AP modes is correct). So if you select excessive rate of descent the aircraft will overspeed, and an excessive rate of climb will cause airspeed to decrease until it reaches minumum. Then you are well on the wrong side of the drag curve and the aircraft will probably be losing height, not climbing.Kevin Hall

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