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HighFlyerPL185

Speed raise on final approach

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Hi,

 

I have this issue where occassionally the speed increases on the approach, causing quite almost a fatal landing. I can't get down to why this is happening, so I thought maybe I could get some help here, as I'm completely puzzled. I have set up the FMC to follow a route between EGGP and EICK. The entire flight went well, the AP followed all waypoints to T/C and down to approach. I have began my descent with VNAV at TOD, and set the altitude knob to 16000 as instructed by the FMC to descent the next waypoint. I have been changing it all the way, until the very final waypoint before the approach. After the final turn, I was aligned with EICK Runway 35, and I took manual control for a manual landing, disabling autopilot using CMD A, disabling VNAV and LNAV and leaving A/T on 152kts as instructed by the FMC for a 15 degree flaps landing.

 

This is the point where things started to get really ugly. The speed suddenly started to raise, although I was descending at a normal rate with Flaps 15. I had my speedbrake set to 2, and I have engaged my spoilers way back actually during descent. The aircraft kept rising in speed, until I reached 198kts at touchdown, and I did not manage to get a soft enough landing for my VA PIREP to be filled. Oh well, forget the hour.

 

I have to admit, PMDG 737 NGX is such an enjoyable aircraft that I absolutely love flying for it's realism, but things like this make it deeply frustrating. I love the amount of simulation involved, and I personally like following the guides and setting the aircraft up, but things like these draw me back from FSX for a longer periods of time. I was thinking what have I done wrong during the descent or the approach, but even my friends can't suss out what it could have been. Or maybe I haven't read the guides properly... I'm sure I have though.

 

Could anyone please point me in the right direction what I could have been doing wrong so the aircraft doesn't behave like that in the future anymore?

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Strange one. I guess N1 just started to rise on it´s own and ignore the target speed for the AT? If you disengage the AT then the throttle behaves normally? Your not accidently pressing TOGO button just before landing or have it mapped to the same key as something else.

 

Does it happen with a higher flap setting on approach. I have never used flaps 15, the speed seems high, especially for EICK, I wouldn´t land at 198kts!, Best to go around, sort the problem and land again.

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Strange one. I guess N1 just started to rise on it´s own and ignore the target speed for the AT? If you disengage the AT then the throttle behaves normally? Your not accidently pressing TOGO button just before landing or have it mapped to the same key as something else.

 

Does it happen with a higher flap setting on approach. I have never used flaps 15, the speed seems high, especially for EICK, I wouldn´t land at 198kts!, Best to go around, sort the problem and land again.

 

Hmm, I don't think TO/GA got engaged. I only pressed Z to disable the AP and then disabled VNAV and LNAV by pressing the buttons. I would have to waste another hour to discover it all :) Guess it's for the best.

 

The speed was correct before the AT disengaged, 152kts at 15 flaps, as specified by approach reference page. Then it started to climb all of a sudden. My throttle was at idle... I'm confused.

 

Is there any chance VNAV could have interfered? I'd like to recall my smooth flight to Bergen earlier on today, landed smoothly, but I used LVL CHG instead of VNAV.

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Pressing Z should be enough, once to disable the AP, again to cancel the warning horn, your VNAV and LNAV shouldn´t have to be pressed after that. Although I doubt that has anything to do with the problem.

 

I have never used VNAV or LVL CHG so close to the ILS before. I usually use vertical speed mode so that I get to the height I want then slow down once levelled off. Then if I can intercept the localiser before the glideslope, I press LOC , then once at the glideslope height, press APP and wait to intercept, then I press Z at anytime from about 1500ft down if I want to land manually and disengaging the AT at the same time or around 500ft latest.

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Pressing Z should be enough, once to disable the AP, again to cancel the warning horn, your VNAV and LNAV shouldn´t have to be pressed after that. Although I doubt that has anything to do with the problem.

 

I have never used VNAV or LVL CHG so close to the ILS before. I usually use vertical speed mode so that I get to the height I want then slow down once levelled off. Then if I can intercept the localiser before the glideslope, I press LOC , then once at the glideslope height, press APP and wait to intercept, then I press Z at anytime from about 1500ft down if I want to land manually and disengaging the AT at the same time or around 500ft latest.

 

I never use ILS actually, only for bad weather conditions, but might be worth a try. Apparently the TO/GA switch is on a screw on the AP panel, is that right? If so, can it be disabled once enabled, or will it carry out the action regardless? I'm thinking whether I could have accidentally slipped my mouse and pressed the TO/GA button, which would certainly be foolish, but I haven't really been touching the A/T switch until the very end. It was set on 152kts.

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disabling autopilot using CMD A, disabling VNAV and LNAV and leaving A/T on 152kts

 

There is a world of hurt contained in those few words, and it's what causing you to botch your approach and landing. As mentioned already, you don't manually select LNAV or VNAV to disable them. If you want to clear the AFDS pitch and roll modes when flying a visual approach, you should cycle both the Captain's and First Officer's F/D switches (turn them both off, then both on again).

 

Secondly, and most importantly, you should disable the A/T at the same time as you disconnect the autopilot. The 737 is not an MD-11, in which you would leave the autothrottle engaged until touchdown; Boeing manual approaches should be flown without automation all the way, hands on control wheel and throttle.

 

For more information, turn to the FCTM, page 5.74 ("Circling Approach - General").

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Most airlines have a policy of no auto throttle if you are flying manually. It makes sense because the airplane doesn't know in advance what you are going to do, so the throttles get behind. You should be manually using the throttle. Also a flaps 15 approach is not normal. You should be at 35 or 40. Flaps 15 is more for emergencies or high winds.

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I think American uses F15 regularly, but for more airlines, it's for mechanical problems. A few airports need it for hot & high go-around.

 

You can leave LNAV-VNAV on the whole while, if you like. There's no need to kill it.

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If you want to cut LNAV/VNAV on a visual approach so that the Flight Director bars don't get distracting, just turn off the Flight Directors themselves. This is a valid and normal thing to do.

 

Turning a MCP mode "Off" without replacing it with anything is a little unusual. If the Autopilot was still in, it would usually cause CWS mode to replace the CMD autopilot. Depending on SOP's of various airlines this is usually not a normal mode.

 

I'd like to suggest using flap 30 or 40 for landing because this will give more drag, and more lift so the approach is slower, and can be slowed down easier.

 

Once you have manual control of the yoke (Autopilot is off and you arn't keeping the flight director centered) it would be recommended to take manual control of the thrust levers too.

 

In this case, it seems the thrust level is far too high on the final approach and flare.

 

The only time Auto-throttle should ever be used in the flare is on a fully coupled auto-land from an ILS approach with the APP mode and autopilot flying the approach. If you use the Auto-Throttle on a visual approach with VNAV/LNAV off and Approach mode not engaged you do run a high risk of the throttles trying to maintain speed in the flare, causing the engines to power up, causing float, and overshoot of the runway.

 

 

At this stage I'd reccomend turning off the autothrottle earlier. and considering flap 30 for approach.

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Just a thought, maybe when you turn off the AT, the idle position of your manual throttle is not being seen. Try giving them a little rock forward/back. Apart from that, something above should fix the problem.

 

It is also a great idea to do both the tutorial flights, the are the easiest and fastest way to learn how to operate the aircraft as close to real life as possible.

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What might be an idea is to dial in the ILS freq and CRS switch the nav display to APP and fly it by hand as you would normaly with the auto throttle on and see if your too high above the GS as if your coming in to steep and with F15 you will have a very hard time slowing it down.

 

When I fly into London city I have this problem as the GS is so step even with F40 and speed breaks out it is very hard to maintain speed often at idle.

 

Also let is know what N1 is at when doing this speed if its still hi.

 

Charlie

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What I've tried is keeping AP awake, with A/T on, but turning off LNAV and VNAV. It worked rather well, but I noticed it would not go down at a certain altitude, and I can't remember what it was. I had 2000 ft on the altitude knob, but ALT HOLD or LVL CHG were not enabled, unless AP is told not to go down past the specified altitude? I had to just kick it off the AP with joystick input and pitch the nose down a bit to return to the glideslope, but all in the end went well.

 

Which is rather weird, because the A/T was on, and although the AP was on, it was later turned off by joystick input, and following that theory, as said above, approach and landing must be made fully AP or fully manual, which wasn't the case this time round, and it did fine.

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What I've tried is keeping AP awake, with A/T on, but turning off LNAV and VNAV.

 

I mean no disrespect, but I must be blunt here by saying you're doing it all wrong. LNAV and VNAV are not meant to be "turned off" during normal operations, but rather should be replaced by different roll and pitch modes as required by the phase of flight you're in and what you're trying to accomplish. For instance, LNAV should be changed to HDG SEL prior to localizer intercept, followed by localizer capture using the VOR/LOC roll mode. Likewise, VNAV segues into the use of LVL CHG (although not strictly necessary), after which APP mode is armed, the glideslope is intercepted, and the G/S pitch mode is activated. This is but one example out of many possibilities, but the basic tenet remains: do not switch off AFDS modes by pressing the associated MCP buttons.

 

Did you read and fly both NGX tutorials, may I ask? A lot of these concepts are explained clearly and concisely in those two documents, and I'd strongly suggest giving them a go if you haven't done so already. Good luck!

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I mean no disrespect, but I must be blunt here by saying you're doing it all wrong. LNAV and VNAV are not meant to be "turned off" during normal operations, but rather should be replaced by different roll and pitch modes as required by the phase of flight you're in and what you're trying to accomplish. For instance, LNAV should be changed to HDG SEL prior to localizer intercept, followed by localizer capture using the VOR/LOC roll mode. Likewise, VNAV segues into the use of LVL CHG (although not strictly necessary), after which APP mode is armed, the glideslope is intercepted, and the G/S pitch mode is activated. This is but one example out of many possibilities, but the basic tenet remains: do not switch off AFDS modes by pressing the associated MCP buttons.

 

Did you read and fly both NGX tutorials, may I ask? A lot of these concepts are explained clearly and concisely in those two documents, and I'd strongly suggest giving them a go if you haven't done so already. Good luck!

 

I read them, however I didn't attempt flying them yet. What steps are therefore ideal to take if I want to fly using autopilot, but land and approach manually with A/T on? Is there a section that focuses on something like this in the tutorial?

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