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ahinterl

757 issues

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I've some issues with the PSS 757:1) VNAV disregards speed constraints during descends (don't know whether this is the case for climbs as well)2) speed only constraints cannot be constructed in the FMC3) LNAV doesn't intercept the planned lateral path aggressively enough4) A/T disregards speed limit below FL100 with flaps extended5) Altitude capture during climb is poor (don't know whether this affects descends as well)To better document the problems, I've collected a reproduceable step-by-step guide (below).Andreas==============================================Flight LPPS-LPMASituation in FS: fair weather, 12:00 local time at LPPS, positioned for takeoff on rwy 36.Plane used: PSS 757-200 PW overwing exits.Aircraft config: ZFW=176184 lbs, fuel=2800 lbs, GW=178984 lbs.Target of flight: start at LPPS rwy 36, go ahead approx. 2NM on radial 007 of VOR SNT while steadily climbing to CRZ ALT 6000, turn left and proceed to FUNOR, then further to VOR FUN where the plane should have descended to 2000, then ahead to user defined waypoint 7 DME from VOR FUN on radial 213 where 1000 ft and 140 kts should be reached; at this point a disconnect of the autopilot/autothrottle is planned to allow for a manual approach to LPMA rwy 05.The plane is positioned at LPPS rwy 36, engines running.To keep things short and easy, I assume that the person following my step-through handles the systems accordingly, so I don't mention many things separately (flaps, gear, lights,...).FMC, POS INIT page:enter LPPS in scratchpadpress LSK2L (ref airport)press LSK6R ("ROUTE>" prompt) to advance to route pageROUTE page:enter LPPS in scratchpadpress LSK1L (ORIGIN)enter LPMA in scratchpadpress LSK1R (DEST)press NEXT PAGE to advance to ROUTE page 2/2enter SNT007/2 into scratchpadpress LSK1Ron "SELECT DESIRED WPT" press LSK1L to select SNT VOR (114.90)enter FUNOR into scratchpadpress LSK2Renter FUN into scratchpadpress LSK3Renter FUN213/7 into scratchpadpress LSK4Rpress LSK6R ("ACTIVATE>" prompt)press EXEC keyquick check route on NDpress LSK6R ("PERF INIT>" prompt)PERF INIT page:right klick LSK1L to enter GR WTenter 1 into scratchpadpress LSK4L (RESERVES)enter 6000 into scratchpadpress LSK1R (CRZ ALT)enter 300 into scratchpadpress LSK2R (COST INDEX)press LSK6R ("TAKEOFF>" prompt)TAKEOFF REF page:enter 15 into scratchpadpress LSK1L (FLAP/ACCEL HT)press LSK1R to LSK3R to confirm v-speedsenter 17 in scratchpadpress LSK4R (CG)set speed bugs according to v-speeds shown on FMCMCDU:set HDG to 007set ALT to 6000arm A/Tswitch on F/Dpress LNAV and VNAV to arm lateral/vertical modes (check on PFD)Again, we proceed in FMC:press LEGS buttonACT RTE 1 LEGS page:I want to be at 2000 at FUN, so:enter 2000 into scratchpadpress LSK3RI want to reduce speed to 140 at FUN213/7, so:enter 140/ into scratchpadpress LSK4R*******************************BOFF! Entry not possible! Why?*******************************To resolve this:press CLR keyadd to "140/" that's still in the scratchpad:enter 1000 into scratchpad (so it reads "140/1000")press LSK4Rpress the EXEC key to make the changes active.press the DEP ARR keyLPPS DEPARTURES page:press LSK2R to select rwy 36press the EXEC key to make the final route activeThe ACT RTE 1 LEGS page shows the automatics would try to pass SNT01 with 325 kts, a clear violation of the "250 below 10000" rule, but let's see...Go ahead: place flaps to 15, check speedbugs, release brakes, advance throttles then TO/GA with a right click on the EPR button on the MCDU.Lift off, retract gear, switch on CMD and watch the magic work (LNAV and VNAV should've taken over in CMD mode).On climb, retract flaps and watch if THR LIM is set properly.Approx. during the turn left towards FUNOR, enough speed should be gained to allow for flaps up.*******************************Watch the speedbug: the FMC sets it to the ECON CLB speed of 325 kts which is wrong since we're still below 10000.*******************************Still during the left turn, the plane should reach 6000, our CRZ ALT.*******************************In addition, the autopilot overshoots our CRZ ALT with approx. 3000 ft v/s.This results in a subsequent -1500 ft v/s dive to regain CRZ ALT.**************************************************************When the end of the left turn approaches. suddenly the FMC lowers the commanded speed to approx. 240 kts (watch the command speed bug).The plane travels at approx. 290 kts here.*******************************Prior to T/D, reset MCP ALT to 800 (watch the FMC message).*******************************During the flight now, watch that LNAV commands an extremely shallow attempt to join the magenta path prior to FUNOR. In fact, there's no indication that the route would be joined until FUNOR at all but instead it seems the automatics are heading directly to FUNOR.*******************************FMC:When passing FUNOR, press the DEP ARR key to define the approach runway:LPMA ARRIVALS page:press LSK1R to select rwy 05press the LEGS keyLEGS page:press the DELETE keypress LSK4L to delete the waypoint CF05 which was inserted automatically during selection of the approach runwaypress LSK4L to downselect waypoint RW05press LSK3L (THEN) to close the ROUTE DISCONTINUITYpress the EXEC key and check the modified route on the NDAt this point, close to FUN, I'd expect the autopilot to do something to reduce speed from now 240 to the 140 required at the waypoint following FUN, FUN02.The distance between the two is only 7 NM, and we're in a descend, so I cannot imagine the plane will be able to bleed off 100 kts...*******************************Passing FUN correctly at 2000 ft, the FMC doesn't correct the target airspeed which is still 240 kts and thus disregards the speed constraint completely.*******************************So, immediate action is required by extending speedbrakes, pressing the SPD button and dialing 140 into the speed window of the MCP. This disconnects VNAV.To further reduce speed, flaps are deployed according to the speed schedule. the plane will further descend and hold at 800 ft.On the FMC APPROACH REF page, select flaps 30.Prior to FUN02, the last usable waypoint along the route, A/T and CMD are disengaged and the final approach flown manually.

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Hi Andreas,An interesting route and final approach. I recall several years ago standing on the terminal roof watching a holiday charter trying to land:1. Overshot2. Touched down then overshot3. Touched down and stayed down with everything out including an anchor.I have just enjoyed flying the route to familiarise myself with your comments - I guess you set it up as an FMC test - and I will fly it again later.I suspect that some of your issues are normal operation of the FMC and others are either limitations in the original FMC software standard or are items in the "not modelled" category. I'll look in detail on my next flight.However, I have no rw experience of this FMC so will pass any issues we cannot resolve on to the experts.Anyway thanks for the route - that landing is a good challenge for us all.RegardsJohn RooumPS Did you really mean to load only 2800 lbs of fuel?

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I will try your flight tomorrow based on the method you have posted. but like John one thing that concerns me right away is the fuel load on a normal flight I am always looking to have approx 3.5kgs of reserve fuel and one of the UK carriers minimum fuel when doing testing is 12,0000lbs. The aircraft with those weights will climb like a rocket and if flown real world it would be unlikely to be flown in VNAV as altitude burts are extremly likely at low level.Watch this space ;-) Steve masson

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Steve,Agree to that! First time I flew the route, I used 28000lbs.One thing specifically - there appears to be a limit on the minimum speed that VNAV will handle. I have noticed since I started looking at the 777 and 757 that if you plan an approach to ILS etc in VNAV and input speeds of say 180 kts for an IP, VNAV will drop out and MCP speed control will be enabled as the speed falls through about 210 kts. I have always assumed that this was a normal FMC function - it seemed sensible as the aircraft would now be being "dirtied up" for landing?John

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Hi John,the flight setup, loading etc. are solely for testing purposes. Guess no plane this size would go that short and low with so little fuel ;-)I just wanted to see how systems react and work. It would need a much more sophisticated route to check out everything possible during an automatic flight (constraints, manual interventions, mode reversions etc.) but I think one day I'll do such a setup too.Approaches like the one to LPPS rwy 05 are the kind of approaches I really like. Yesterday the landing worked fine with the Lago MD80, next time I'll try to do this with the LDS 767 (let's see how big the plane can be without falling into the sea)...Andreas

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Hi Andrea, Flying in to Madeira is always a challenge and this had to be one of the quickest flights I have made to the Island. To help answer some of your questions I need to point to some of the limitations of the real 757 FMC that have been modeled by PSS, the FMC is at the end of that day a computer and no substitute for good airmanship. The 757 is now 25yrs old and in many ways is old technology and with many different versions of the FMC software in use today with the airlines. A lot of the functions we have become used to in flight sim in the 747 and other models are just not present in the 757. I realise this was a test flight to see how the FMC, LNAV and VNAV behaved on a flight you have done before, but selecting the 757 and using low fuel loads and weights created an issue straight away, one you identified and I witness as the aircraft burst the cruise altitude by 245ft. Why did this happen? Basically the aircraft climbs like a rocket empty and it takes a degree of skill flying the real aircraft to stop it under those circumstances burst altitude restrictions and speed restriction for the different configurations. In practice the aircraft would not be flown in VNAV at these lower heights and more trust would be put in airmanship rather than old computer software.You raised a number of valid points about the FMC. 1. Econ speed was above the 250/10000 restriction 2. On over speed routing to FUNOR 3. The aircraft ignored speed restrictions placed on the legs page for FUN213/7 and it wouldn

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Steve,Thanks for the above - a great summary - as you, I was also thinking "airmanship" before I got past the fuel loading stage.I think the lesson here is that the FMC is no more than a tool for the pilots to use to reduce their workload and help to achieve a more efficient flight. The crew should control the FMC, not vice versa. They should not expect the FMC to do the impossible and should be prepared to change/over-ride the FMC to achieve what they want from the flight.Perhaps something for PSS if ever they decide to update the tutorial.RegardsJohn

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Thank you Steve for your explanations. I wish I had an original AOM for the 757 in some form (pdf,...)...Andreas

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