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Descent planning & FMC

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HelloI've recently experienced a couple of times when descending,a message is received on the FMC "STEEP DESCENT AFTER *****". Upon reaching this point the aircraft plummets from the sky!!!!Is this pilot error? Bad descent planning etc?Thanks Alan W

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Full names here please Alan.Look on the LEGS page to find your answer. It will probably show altitude constraints that are difficult to achieve.The 737 doesn't do well with conditional constraints, and this can also lead to the steep descent problem. For example, if you have an AT OR BELOW 15000 and ten miles later you are supposed to be at 10,000 you should be crossing the AT OR BELOW somewhere around 12500 but the 737 will probably cross it at 15000 setting up the steep descent of 500ft/nm which is very difficult to achieve and will put peanuts on the ceiling.The 747 is better but the only one I use verticle navigation on descents is the MD11. It's PROF is amazing. I expect the new 737 to solve this problem.


Dan Downs KCRP

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Thanks for the reply. Looks like I need to improve my descent planning skills.CheersAlan Watson

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Practice makes perfect :( The more you fly the more you'll understand the descent logic of the FMC. I normally use V/S mode for descent with the VNAV indicator on the ND as a reference.

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HelloI've recently experienced a couple of times when descending,a message is received on the FMC "STEEP DESCENT AFTER *****". Upon reaching this point the aircraft plummets from the sky!!!!Is this pilot error? Bad descent planning etc?Thanks Alan W
Alan,Some STARS are just designed poorly, in my opinion. They'll give you a "Steep Descent" warning every time as you're taxiing for departure. As long as the airspeed stays reasonable, I wouldn't worry. If the jet won't make a crossing, think about speed brakes or telling ATC that you'll be unable Altitude/Airspeed at fix XXXXX. That generally works in the USA, I don't know how well that statement would be received elsewhere.Like Onur said, keep the VNAV indicator in mind.Have fun.

Matt Cee

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I concur with Dan- the 747 is great in VNAV and the MD-11 is superb. The way around the 737 issue is to hard code the STAR- change the conditionals to defined crossings. Using this technique on MOST STARS allows for VNAV descent. Remember- the FMC is strategic, the MCP tactical. Utilize the hard crossings to descend AND to slow the aeroplane.RW 737 drivers know the folly of being high AND fast in the baby Boeing :)Countless times I have descended in LNAV/VNAV to the FAF and intercepted localizer/GS in VNAV/LNAV- beautiful.Obviously these are at remote locales....with no ATC. Still, it is a lovely thing with it occurs :)Fully understanding the intracasies of the automatics has been a rewarding endeavour. The 747-400's FMC is almost perfect (with a few wish list items outstanding).


Best-

Carl Avari-Cooper

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On the 737s I don't hard code the altitudes everywhere but take advantage of FLCH and the descent arcs on the ND changing to V/S for control. This allows me to predict where I'll be for the crossing restrictions at the next or even further waypoint displaying on the ND. Although constant descent approaches are being tested they are not too common yet.There was an article in Airways magazine a few months back where the r/w Captain deviated from the FMC planned descent and came right out at the assigned altitudes especially for final where the FMC advisement would have placed him too high. He stated a couple of his "rules of thumb" in it. His copilot gave him questioning looks when the Captain commanded an early descent but was surprised at the result.I think he requested also and obtained a lower cruising altitude than "optimum" before entering the approach phase.I have not done this but I think winds can be entered on the descent page if I recall giving the FMC a better idea of the ground speed to expect in reaching any restrictions and adjusting airspeed accordingly. These winds will often change as you descend into the lower altitudes than those experienced at cruise.

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The 737 has always had a bit of a reputation for being hard to slow down and get down, some of which was as a result of problems with the intake spinner on the CFM, which was known to exacerbate water ingestion into the engine in heavy rain when on low engine thrust settings. The problem has on occasion resulted in engine flame outs and, once or twice, dire consequences, including one aircraft (a Garuda Indonesia 737-300) that had to belly land without flaps in the Benjawang Solo river in 1989 after both engines flamed out and refused to restart (one of the cabin crew was killed in the landing when the tail broke off and she was thrown out but to be honest, the crew did well under the circumstances, despite that tragedy). This was one of a number of flame out incidents on the 737 in the late 80s and early 90s.Until the spinner on the CFM was redesigned with little vanes on it to redistribute the water, Boeing had advised pilots to keep the throttle settings on the high side when coming down through clouds in an official communique, where they suggested a minimum RPM of 43 percent and also advised pilots to not use the autothrottle in rainy conditions on a descent, which of course makes meeting descent restrictions tricky, so it is not unknown for pilots of 737s to use some unconventional methods on occasion, including dropping the landing gear to slow them down so they can maintain high thrust settings. As noted, the CFM's spinner was redesigned to address the problem, but it is worth pointing out that the problem has occurred on CFM engines on the 737 which do have have the redesigned spinner, so a prudent 737 driver will probably keep the revs up in rainy/cloudy conditions, rather than try and make a tight descent restriction with nothing on the clock but the maker's name, and risk a deafening silence.Al


Alan Bradbury

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The 737 has always had a bit of a reputation for being hard to slow down and get down, some of which was as a result of problems with the intake spinner on the CFM, which was known to exacerbate water ingestion into the engine in heavy rain when on low engine thrust settings. The problem has on occasion resulted in engine flame outs and, once or twice, dire consequences, including one aircraft (a Garuda Indonesia 737-300) that had to belly land without flaps in the Benjawang Solo river in 1989 after both engines flamed out and refused to restart (one of the cabin crew was killed in the landing when the tail broke off and she was thrown out but to be honest, the crew did well under the circumstances, despite that tragedy). This was one of a number of flame out incidents on the 737 in the late 80s and early 90s.Until the spinner on the CFM was redesigned with little vanes on it to redistribute the water, Boeing had advised pilots to keep the throttle settings on the high side when coming down through clouds in an official communique, where they suggested a minimum RPM of 43 percent and also advised pilots to not use the autothrottle in rainy conditions on a descent, which of course makes meeting descent restrictions tricky, so it is not unknown for pilots of 737s to use some unconventional methods on occasion, including dropping the landing gear to slow them down so they can maintain high thrust settings. As noted, the CFM's spinner was redesigned to address the problem, but it is worth pointing out that the problem has occurred on CFM engines on the 737 which do have have the redesigned spinner, so a prudent 737 driver will probably keep the revs up in rainy/cloudy conditions, rather than try and make a tight descent restriction with nothing on the clock but the maker's name, and risk a deafening silence.Al
One of the first things they teach in B737 ground school is: The 737 can go down and it can slow down, but it can't go down AND slow down.I need to do some study of the history of the 737, I haven't heard of the flame-outs. I've only been on the 737 since 2007, but I've never seen anyone take extra precautions for descents in icing conditions other than to maybe tell the FMC descent forecast that TAI would be on from ALT XXX to ALT YYY. I do know the flight idle N2 with TAI on is higher than with TAI off (thus the ability to insert that alts on the descent forecast). I guess that was part of the modifications along with the spinner redesigns.Good discussion.

Matt Cee

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