Bonjour,I would like to have some precison on one of your previous thread concerning non precison approach.First I would like to say that all your answers are always very informative and are a goldmine.In one of your thread you said that at MDA you start to level off (ALT/HOLD mode engaged),and set the MCP to the missed approach altitude and then if visual disconnect A/P, A/T 50 feet below MDA.I would like to understand how is it possible to go below MDA with A/P on as if you do nothing the plane will stay at the MDA altitude.The only way I found to descent below MDA is as soon ALT/HOLD engaged , I set very quickly the MCP to Missed approach altitude and then swith to V/S mode -700 ft/mIs it the correct procedure to reach 50 feet below MDA with A/P on ?I have two more questions 1 Is it always the PF who disconnect A/P and A/T ? 2 On the Yoke is there a button to disconnect A/P and another one for A/TMerci beaucoupBest regardsClaude

Not being HPSOV or another specialist, I cant answer all your questions but Ill have a go at some:"I would like to understand how is it possible to go below MDA with A/P on as if you do nothing the plane will stay at the MDA altitude."If youre on a non precision approach, you wont use the autopilot at that stage anymore. Disable the autopilot and the flight director, and fly visually. If you are really desperate to use the autopilot (for one reason or the other), you can of course use vertical speed to descend, but that could become dangerous, especially if the altitude box displays an altitude above your current altitude (you would descend right into the terrain if you were distracted). From the MDA on, the pilot has to be completely "in the loop", thus no a/p."1 Is it always the PF who disconnect A/P and A/T ?"I guess that could be an airline policy thingy, but the ones I have seen, it was the PF who disconnected both the A/P and the A/T. With other airlines, he might just call for it."2 On the Yoke is there a button to disconnect A/P and another one for A/T"You can disconnect the A/P with a switch on the yoke, and there is a switch on both sides of the thrust levers with which you can disconnect the auto throttle.Hope this answered some of your questions.Regards,Mark

I think thats spot on Marcom....i think! tehe :)

Mark,I sorry, I have to disagree on your statement about the use of autopilot for non-prec approaches. In the operation I am familiar with automatic flight the use of VNAV or V/S mode IS the preferred method of flying non-prec approaches. The idea is to reduce the total workload and use the added safety margin provided by the automatic systems. Once you are VMC you may choose to hand fly the rest of the way. These days most non-prec approaches are flown using the constant angle method rather than the "dive and drive" method. Recently rules in some countries (Canada at least) have been amended to permit descent below MDA (in the same manner that this is permited for Precision approaches) on these type of approaches. In this case, assuming you were VMC at MDA, you could continue the descent on autopilot until 50 ft below MDA (again a common procedure on prec approaches)Kevin in CYOW

Marc,OK I will try this again with a little proofreading this time. Sorry, but I have to disagree with you on your statement about the use of autopilot for non-prec approaches. In the operation I am most familiar with, automatic flight and the use of VNAV or V/S mode IS the preferred method of flying non-prec approaches. The idea is to reduce the total workload and use the added safety margin provided by the automatic systems. Once you are VMC you may choose to hand fly the rest of the way. These days most non-prec approaches are flown using the constant angle method rather than the "dive and drive" method. Recently rule changes in some countries (Canada at least) permit descent below MDA (in the same manner that this is permited for Precision approaches) on these type of approaches. In this case, assuming you were VMC at MDA, you could continue the descent on autopilot until 50 ft below MDA (again a common procedure on prec approaches)Kevin in CYOW

Hi Kevin,What you wrote in paragraph 1 was what I meant in my statement (maybe it came over incorrectly)So, I agree, using the A/P and A/T is a big help until you reach MDA. Just make sure the PF is in "raw mode", especially when flying LNAV to make sure everything the information agrees.To be honest, I don`t really understand the reason behind paragraph 2. So if you reach the MDA, you are allowed to descend another 50 ft on A/P if you are in VMC. What`s the point of this? Whats the difference if you disconnect the A/P in lets say 1000 ft as opposed to 1050 ft?I didn`t know that airlines used this procedure, so sorry for giving misleading info above. In this case I would not know of a "clean" way of descending exactly 50 ft below MDA, being in altitude hold and having the missed approach altitude in the box. You could certainly use vertical speed, but then timing it to nail the 50 ft correctly would seem as a waste of time ( at least to me, as I don`t quite understand the logic behind descending 50 ft below MDA)Regards,Mark

I think you misunderstood the original question and marcom's reply. Marcom is correct in his assessment. Claude's question pertains to what to do once you have decided to descend out of the MDA, not about whether an autopilot should be used on a non-precision approach. When leaving the MDA on a non-precision approach, whether you dove and drove or had FMS slope help, it's best to hand fly it yourself since it becomes a purely visual operation at that point.

It's just that you are allowed to keep the autopilot on until 50' below MDA, not level off 50' below MDA. So at MDA with runway environment in sight, on A/P crank in a V/S of -700 or 800 and be sure to disconnect by 50' below MDA.

I think the question has been answered, but I'll just cover it from my angle!(Bearing in mind I'm on a months leave and have had to dig out the books for this one :-P).For a 'normal' non-precision approach the preferred modes are LNAV and VNAV, if either of these modes is not producing an acceptable result (due to inaccurate map etc) then you can revert to HDG SEL and V/S. For a VOR approach it is normally the PNF that will have expanded VOR on his display, the theory being the PF should have the MAP display to aid his situational awareness.Once upon a time you would dive down to the MDA, level off and wait to get visual, once visual commence a descent. I assume this is why there is a limitation on the autopilot of 50ft below MDA.The thinking has changed to a constant 3 degree approach, even on non-precision approaches (infact in the last few months they've been changing our Jeppesen plates here to depict only a 3 degree approach for non-precision, instead of the normal step-down profile).So these days you simply fly your 3 degree profile in VNAV, or if thats not working in V/S using the distance/altitude table on the chart, or the one you've worked out for yourself.You set the closest altitude below the MDA in the altitude window, approaching MDA you reduce your v/s and select ALT HOLD such that the aircraft will not descend below MDA, and set the missed approach altitude once in ALT HOLD.If you're visual you disconnect autopilot and autothrottle, and switch the flight directors off and perform a manual, visual landing. Bearing in mind this must be done QUICKLY or you will end up very high on profile after levelling off (in reality you disconnect the autopilot the second you get visual, to prevent the ALT HOLD from taking you above the profile).If you're not visual then you hit the go-around switch and round you go.So to answer your question the 50ft below MDA is the limitation on the autopilot, not what you actually do!!And your other questions.... As far as I know its always the PF who does it, although I guess it could work for him to call for it if he wanted to.And the yoke has a switch for A/P disconnect, and each thrust lever has a switch for A/T disconnect.