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Guest iain williams

Approach control - speed & thrust?

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Guest iain williams

Hello: Yes I know this has been discussed before - but the more I read the more I seem to get confused. I need someone to put me right on this one - please. By the way, I fly the 737.Speed is controlled by pitch and rate of decent is controlled by thrust - is this correct?I have read so many differing views on this.One article said: For normal cruising, etc, pitch controls speed (sounds good to me). The artcle then went on to say before glideslope capture speed is controlled by pitch and rate of decent controlled by thrust (I agree). It then said that after glideslope capture, the opposite applies - in other words speed is controlled by thrust and rate of decent is controlled by pitch. I hope you can understand my confusion.I was always thought the former (makes with ****) was the correct way to land a jet with the pitch on appraoch set to about 5 degrees.I welcome any help. Perhaps some kind person can succinctly right the methodology in point form.Many thanks..............Iain

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Guest adesmith

The truth of the situation is that pitch and power affect both speed and descent profile.When you have the aircraft on a stable descent profile close to the glide path then I strongly recommend the "power and point" technique whereby you point the nose where you want it (pitch for glideslope) and adjust the power for the required speed.If the power is set at the right level for the desried speed then small adjustments of pitch to compensate for small glidepath deviations will not give significant speed fluctuations.If, on the other hand, you are significantly off the nominal glide path then I suggest pitch for speed and power for path. Holding the pitch angle and varying the thrust will produce a slight speed change but will produce a significant vertical path change.Let the autopliot/autothrust fly a few ILS approaches and note the power settings and pitch settings it uses.THEN try it yourself using those settings and make SMALL adjustments to achieve accurately what you want.The important thing on an ILS is to make small adjustments.

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Guest D17S

The autopilot's Glideslope uses Thrust to control airspeed and uses Pitch to control the rate of descent, or path. Notice the far left FMA (thrust) is in "SPD" while the far right FMA (pitch) is in "GS". But the the autopilot's FLCH does the opposite. FLCH uses Thrust to control rate of descent/climb and Pitch to control speed. Both work. You are sure right about differing views about how a human should approach this. I'm confused already! I flew the MD11 sim and the Boeing instructor told me to use thrust control my rate of descent (path) and maintain my speed with pitch. What do I know? Sounded good to me. We only crashed once he shut down 2 engines and made me (try) to land with one. But I found myself cheating a bit. Doing this takes a LOT of practice. For instance, I'd be a bit high, but on-speed. I'd pull back thrust to increase my ROD, but I'd slow down. Then, I would compensate by pitching down a bit to pickup speed and sink a bit below GS . . . and the hunt was on! Up and down, too fast then too slow. Ya know what? I think the basic idea is like the instructor said, but actually they need to work together, in unison.For instance, if you mouch thrust back to sink just a tad faster, you better be touching the nose over a bit to maintain speed too. Notice I'm using the words "mouch," "touch," and "tad." Once you are on speed and path, the control inputs are SO delicate, almost always in combination and coordinated unison. "Pull em back to dive" or "Pitch up to slow down" might be considered just the basic idea. Gotta open a door? Yes, a heavy footed boot will work! It's more the thought you have in mind. It's probably more about that "coordinated unison" idea, . . . because if you compensate with one, you are going to throw the other off. It's a gentle dance between the two. Always both. So is it pitch for speed or path? Thrust for path of speed? The expert will tell you what they were taught years ago, but to them, it's like ice skating. How do you explain to another human being how to balance a foot directly over a 1/4 inch blade?

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Guest iain williams

Thanks Sam and AS.I found this in a jet flight manual.DESCENT FROM ALTITUDEReduce power to reduce airspeed and alter pitch to desired descent angle.APPROACHUse a steady pitch and change power/thrust to control airspeed.You must increase thrust to descend at a slower rate, and you must reduce thrust to descend at a faster rate. (in other words, set your pitch, then use the throttle to control vertical speed and descent path)CLIMBING & CRUISINGMaintain full power and use pitch to control airspeedYour correct Sam in that it is theory - I find I can do it OK on the sim, but then when I think about it (as I want to ensure I'm doing it correctly) I start to become confused. Too many ins and outs for my liking :)As for replicating the N numbers whilst doing an autoland. Good idea if your aircraft weight is the same for every flight - but I continually alter loads & configurations.Cheers & fun flying....IAIN

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Both are correct depending on what regime of flight you are in. For example if you are descending from altitude at a given speed and you want to reduce the rate of descent you would apply some power. In order to maintain the speed you would raise the nose. Here thrust is regulating rate of descent and pitch is maintaing the speed. You could raise the nose (speed would want to reduce) and then incease power to maintain the speed (this way pitch is controlling rate of descent and thrust is controlling speed). They are interchangeable. However on an approach ie. ILS the correct procedure is to maintain the descent path with pitch and speed with power. This is particularly important in turbulent conditions especially with windshear. If you encountered significant negative shear (ie. sudden loss of headwind) the first action is to agressively apply thrust. What would happen here if you tried to correct a decaying airspeed situation with pitch? The aircraft is loosing speed (lift) and if you were to correct this using pitch (pole forward) then this would cause more loss of lift (reduced angle of attack) and a large rate of descent would occur. This is precisely what you dont want to happen in a jet at low level (on approach). You need to have the speed/thrust action as an automatic response that can be done without thinking. What you do at altitude is really irrelevant as it is not critical.CheersSteve


Cheers

Steve Hall

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Guest iain williams

When your coming in completely manual, and the flight director is on, should you use it to maintain your pitch - then use the thrust up and down to maintain a suitable decent rate?Thank you.....IAIN

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my two cents... briefly: Performance = Atitude + PowerThere is a relationship between pitch and power. The key to a successful final approach is being stable, in this regime then "minor" adjustments are made by pitch to control speed and power to control descent rate. I have been in many situations where the minor adjustment is insufficient, for example a wind shift may require adjustments to both pitch and power quite quickly to maintain desired speed and path. Don't expect to master this in only a few hours, I have 20 yrs of actual instrument time and I am still learning.


Dan Downs KCRP

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What I think needs to be addressed beyond the basics and not mentioned in posts so far is the delayed in response time to thrust control which would be critical in final. Therefore to follow the glide path especially in a bumpy path due to vertical currents in that stage pitch would be the dominant control for best response.For FLCHG commands in descent those are more of an immediate nature and I've read and it appears that most practise and idle thrust in this case with pitch controling rate of descent for within speed safety parameters. This has to do with altitude restrictions V/S required to comply with ATC.


Ron Ginsberg
KMSP Minnesota, Land of 10,000 Puddles
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Guest iain williams

Iain here (thread starter).Some excellent & informative replies. It seems there is no right or wrong way and methodology depends on the variability of each approach situation. I have successfully used pitch/thrust on many occasions - and by and large the easiest approaches I've made are when I've configured the aircraft for landing, with flaps and speed by the outer marker - then I've used both pitch and thrust to maintain glideslope and rate of descent.My initial concern was whether I was "running" the approach correctly. From the threrad it would appear that I am. Many thanks.....Iain

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Guest iain williams

Hello againOK I've been practicing and getting better - trouble is finding the time to get consistent practice.Can you tell me if this is the more or less THE correct method for approaching.At glideslope capture (but not descent) I ensure I am VREF plus 15 with appropiate flaps out. This is about 12 mile out.When I intercept the glideslope, I alter the aircraft configuration to VREF +5 with flaps in landing configuration (25 or 30 for standard approaches).I set my pitch at roughly 5 degrees nose up and use throttle to maintain glideslop/vertical speed. As I juggle the thrust up and down a bit I may alter the pitch a little - but not too much.This seems to work well for me - question is is it CORRECT?Also, is it OK to be at VREF + 15 at 12 mile out? Doing the approach this way I guess I am using both pitch and thrust to maintain glideslope.Many thanks. I hope others are learning from this thread as welll.....IAIN

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If it works for you then it is correct. In terms of correct real world handling a heavy jet it is wrong. On approach thrust is speed and pitch is ROD. See my previous post. If you dont do this you are setting yourself up for a potential disaster.As for the Vref speed you can be Vref +5 at 12 ml out if you wish.The approach configuration ie. speed, flap and gear is largely dependant on the weather/traffic conditions at the time. If the aircraft is on approach to an airfield that has weather about the approach minima than it is usual to fly it at the landing flap configuration from (or shortly after) the initial approach fix (tip over point). If the weather is fine then, in the interests of reduced fuel consumption and or traffic build up, the approach is flown in a staged fashion. In my company it is as follows:Flap 20 and gear down must be selected by 1500 ft AGL. Landing flap must be selected by 1000ft AGL. Aircraft must be stable at 1000ft IMC or 500ft VMC. That is it must be spooled up, on course, on profile and the speed -5 to +10 of targeted speed. If not a go-around is mandatory.Hope this helps.CheersSteve


Cheers

Steve Hall

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Guest iain williams

Thanks Steve - obviously I have a problem with this (in my mind) and I will need to work on it. Back to square 1. Yes the post was very helpful - thanks......Iain (very junior pilot) lol

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