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garydpoole

A question on the real world use of LNAV and VNAV

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HiI recently acquired WorldAirRoutes excellent DVD featuring Helios Airways 737-800's. Watching it, i was slightly supprised at how little use LNAV and VNAV got. It appeared that the SID's were flown manually using the FD not LNAV and VNAV would only get selected in the final stages of the climb to the cruising altitude - level changes being used upto that point.This differs to the various tutorials that i've read that suggest engaging both LNAV and VNAV quite early on in the flight. I suspect that level changes are used due to ATC vectoring to different altitudes, but i'm guessing - any comments on the subject would be appreciated.RegardsGary

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Hi Gary,LNAV (or VNAV) and flight director are not things that exclude each other.As far as I know, it's forbidden to fly SID's using raw data on the 737 (at least it is like that on the 757/767). Most likely they have flown the aircraft manually with LNAV engaged. Thus the Autoflight director system has been providing the flight director with the desired commands to maintain that lateral path LNAV is commanding. Simpler, the pilot has flown the aircraft according to AFDS commands.Or, if they were given radar vectors due to traffic congestion soon after departure, in that case they would be using HDG SEL and also most probably flying manually initially.VNAV is another thing though. Many airlines have SOPs that do not include using VNAV all the way from acceleration altitude. Instead, they will use FL CH to accelerate the aircraft to clean maneuvering speed to get best possible climb gradient, and/or to follow noise abatement proccedures. Of course same result can be achieved with VNAV by programming a modified climb into the FMC. However, FL CH is much more versatile and quicker to use in situations where pilot task load is high. Same goes for approaches. However, some airlines, such as Finnair, will use VNAV (and LNAV) all the time when possible in their 757s. Makes for economical flying. Many times it's a question of merely preferance and/or airline standard operating procedures. However, often a question can be raised as well: why use non-economical modes when not neccessary ?As a general good practise to get used to in the sim is that you engage LNAV at 400ft AAL and VNAV at acceleration altitude (usually 1000 or 1500ft AAL). If needed you can fly the departure using HDG SEL and FL CH, but they are non-standard procedures, not recommended by Mr. Boeing in the official documents. ;)cheersTero

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Aw, why would flying a SID on raw data be forbidden? It's not forbidden on the 757/767 either. The 757/767/744/777 don't have a VOR tracking mode in the AFDS anymore, but SIDs are much more than just radials.Only local regulations may say that you should engage the autopilot as soon as practicable and may have RNAV-only SIDs.

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Iz,How strange.I heard this from a UAL 767 pilot. Of course it sounded strange to me also, and yes, what you say about SIDs is true. But I was told that it's a "risk" flying SIDs with raw data, but maybe I misunderstood what he meant. Considering that MOST SIDs do have vor radials (non-RNAV) in them, and taking into account that the various Boeings don't have a VOR tracking mode in them, it would seem rather awkward to then go and fly the SID with raw data. But apparently it IS allowed, then ?Tero

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