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Guest tmetzinger

LVL CHG and VNAV with Autothrottle disengaged

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Guest tmetzinger

Got a quick question.In the real airplane, with the autothrottle off, is it possible to engage the VNAV or LVL CHG modes?In the PMDG, the modes will engage, and I'm not sure if this is correct behavior.Of course the modes don't work "properly" - I just wonder if they're inhibited in real life, or if it's possible for the pilot to be the "human autothrottle" and set thrust accordingly and have the appropriate flight path result.The related question is whether these modes are EVER used in real life (i.e. Southwest) when the autothrottles are disabled.I'm working on the chapter for flying "Southwest Style" and I'm advocating the following techniques, and wondering if they're accurate:Takeoff - hit the TOGA button, and manually advance the throttle to the indicated N1 setting. This gives you useful pitch guidance on the PFDClimb - Autopilot ON, set thrust to Climb N1, and use VS to give you the appropriate airspeed. Remember to increase the thrust as you climb!Cruise - ALT HOLD and set thrust for desired speed or fuel flowDescent - use the DES page to indicate Top of Descent, and pull the throttles to idle and set VS to match that given on DES page, so you meet restrictions/constraints.

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Guest meyomyx

Probably one for Brad but .......The MCP controls the Flight Director via the FDCs - so engaging these modes should still command the FDC's to calculate an appropriate envolope for display on the Flight Directors? (regardless of autopilot/autothrottle)Taking LVL CHG - if climb thrust is not available (ie autothrottles disconnected), would not the FD indicate a marginal climb (in terms of pitch) for the given thrust? or indeed level flight.VNAV passes pitch and thrust to the FMC - perhaps the above applies?It would seem logical that either mode would tell you there's a problem if the "goal" became impossible.

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Guest tmetzinger

Well I did some experimenting, and found that on the sim at least, LVL CHG works as expected - it will use pitch to maintain the selected speed, and the thrust you set affects the rate of climb or descent, at least until you don't have enough thrust to maintain a minimum rate of climb - then it raises the nose to maintain the minimum rate of climb.Speaking to a real SWA pilot over in the SimSouthwest forum, I learned that they do use LVL CHG for the climb, and do use the FMC's calculated path to adjust VS on descent.It's a nice change from the full auto flight, I must say. Still takes some practice for the non precision approaches, getting the VS just right for the descent profile.

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Guest tmetzinger

I believe that southwest orginally disconnected the autothrottles for the sake of fleet commonality - the -200s they started with didn't have any, and they were able to save money on pilot training by making the fleet as similar as possible.Now that the last -200 is retired, I expect that they'll have to look at the relative costs of implementing autothrottles (maintenance and training) versus the potential savings (reduced fuel usage) that autothrottles might generate.From a piloting standpoint, the few SWA pilots I've spoken to said they felt that not having autothrottles kept them more "in touch" with the aircraft, since every change in weight, altitude, etc, required pilot action on the throttles.

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Guest meyomyx

Now there's a debate.To quote Richard L Collins;"Autothrottles are one of the greatest pilot workload relievers ever invented"

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>"Autothrottles are one of the greatest pilot workload>relievers ever invented"I saw this statemnt myself but I wonder to what extent it is true. Richard is known for some hyperboles once in a while. ;-)Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2


Michael J.

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Guest tmetzinger

>Now there's a debate.>>To quote Richard L Collins;>>"Autothrottles are one of the greatest pilot workload>relievers ever invented"no debate... it's certainly a workload reducer... and it also "separates" the pilot from the airplane. Can be good or bad for safety, depending on how it's implementedAs I said, SWA's reasoning was mostly economic, and it will be interesting to see how they see it in a fleet where all the airplanes now have the capability.

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