744 Cruise Phase - Loss of Airspeed/Altitude

[jeffhunter 0]

It has been discussed at length. What happens, usually associated with changing Wx or turbulence, is that you end up in an excessively nose-high attitude (say 10 deg or so). This results in so much drag that you lose airspeed, which leads to higher pitch attitude and so on and so on....The solution (which actually works), is to change your thrust settings from CRZ to either CLB or CON. This will get you back to the correct attitude/speed. Once established - you can go back to CRZ thrust.This makes it a bit hard on a long flight, because you have to keep an eye on things, but it beats pancaking in at -10000 fpm!regards,Jeff

[744pilot 0]

funny thing. On my flight as described above I used CON thrust and there was no wind change at all. It was just like the air was realy thin at my alt. and so causing not enough lift althroug the TAT was showing -12.So, are you still keeping your theory? ;)

[Guest lou ross]

KMIA-KSEA at FL350???louross.

[opherben 0]

I never suggested to not use VNAV, but rather to avoid lacking excess thrust by setting the thrust limitation to CON during cruise.>Ive just completed a flight from KLAX to PHNL.>>As suggested by Opher when i reached the Cruise altitude I>deselcted VNAV and selected SPD with Mach set to 0.86. I>managed to complete the flight without any trouble. On a>couple of occasions I actually noticed the airspeed start to>bleed off however the engines responded by spooling up very>quickly, and to a higher N1 % than I believe would have been>the case under VNAV, with the result that the airspeed quickly>recovered to the selected value and no altitude loss was>encountered.>>I will try another longer flight sometime this week (if i can>find the time) and report my results but it looks as if the>problem may be overcome, as a short term fix, by flying the>Cruise phase under SPD and not VNAV.>>Still dont know why it is happening mind you. I also fly long>haul, trans oceanic, flights routinely with the LDS767 and>have never had any problem at all.>>Regards>Graeme

[opherben 0]

Ramon,from your description quoted below, it is evident that your flight was problematic, but I found no clue in that description to help me figure out why it happened. I explained professionally what I saw happening on my flights, and what I use to solve it until the aircraft manufacturer realizes this and makes appropriate design changes. Aircraft performance and handling are part of my expertise, and my suggestion is far more than just a theory.I see no deviation from normal a cruise TAT of minus 12.---quote---This is my report of a flight from WSSS to EDDF.Remember I don't have the registered version of FSUIPC.Planned a step climb after each 2000 ft and my initial altitude was FL300 which was the exact OPT. Alt for my weight (nearly 398 Tons) when I finaly got to FL360 around OIII (2/3 of flight past) My troubles started. After a while, flying at FL180, I was able to fly to FL380 for the new Opt alt. but nearly ended up with 6 Tons of fuel remaining at EDDF. I had 235 passengers onboard with around 25 Tons of freight. ASVE is my weather program. So, once again, I and Greame ask the same question again: Wat's causing it?----------->funny thing. On my flight as described above I used CON>thrust and there was no wind change at all. It was just like>the air was realy thin at my alt. and so causing not enough>lift althroug the TAT was showing -12.>So, are you still keeping your theory? ;)

[Guest Graeme Paterson]

OpherMy mistake - misunderstood your suggestion.I have just flown the return leg PHNL to KSFO and, as suggested, set the Thrust Limit to CON under VNAV when i reached Cruise altitude - I deliberately chose to fly at a level higher than OPT but slightly lower than MAX.Anyway I encountered an airspeed loss and increasing nose high attitude situation but, thankfully, the aircraft managed to recover itself on this occasion by increasing Thrust to the limit. I have to say then based on this evidence it does look like your suggestion works.Im still puzzled mind you why i never notice anything like this with the LDS 767.Anyway thanks for your help. Hopefully this problem may be sorted out properly some time.Graeme

[opherben 0]

Hi Graeme,glad it helped.I have no way to fly these real B744 and B763, and compare to their simulations, but believe that the PMDG B744 modeling quality is the reason for some of its problems.It is more difficult for me to troubleshoot this further, since PMDG had not responded directly to my reports of two performance issues, the first being the lack of sufficient excess thrust at OPT ALT cruise, and the second being excessive fuel flow, compared to FMC prediction.I urge you to look into the rate at which weather changes are input from your weather engine to FS9, and limit it at both the engine and FSUIPC settings.Best regards!>Opher>>My mistake - misunderstood your suggestion.>>I have just flown the return leg PHNL to KSFO and, as>suggested, set the Thrust Limit to CON under VNAV when i>reached Cruise altitude - I deliberately chose to fly at a>level higher than OPT but slightly lower than MAX.>>Anyway I encountered an airspeed loss and increasing nose high>attitude situation but, thankfully, the aircraft managed to>recover itself on this occasion by increasing Thrust to the>limit. I have to say then based on this evidence it does look>like your suggestion works.>>Im still puzzled mind you why i never notice anything like>this with the LDS 767.>>Anyway thanks for your help. Hopefully this problem may be>sorted out properly some time.>>Graeme>

[Guest Darren Howie]

Firstly do you have VAS points enabled in AS5?If so immediately turn them off as theyhave the ability to supply enough sink to bring down the PMDG 744 and they are not accurate at least at altitude anyway.Secondly pay for the FSUIPC upgrade and adjust the wind change values to reduce the rate of wind change direction and speed.With those adjustments you will still find you can get 30-40kt decreases in speed but not so much that you get on the back side of the drag curve.The amount of excess thrust available at high FL's is very small at heavy weights on the 400 so supervision is required just like in real life.The 763 has a fair amount of excess thrust so doesnt suffer to the same degree as the 744.By eliminating the VAS points and adjusting FSUIPC settings its pretty easy to overcome the issue's you have mentioned.But the aeroplane still requires some vigilance just like the real world.Darren

[Guest pablo_gsxr750_aus]

HI there to all...I too have been dealing with sudden ALT & SPD reductions whilst attempting my 1st longhaul from KJFK to EGKK, especially at 1 hour airbourne time, FL 29,000, SPD 0.826. This problem seems to be nothing else other than the obvious effects of, winds aloft, the jet streams encounted on flights going West to East and finally an FMC indicated tail wind of 140 degrees at 49 knots.I've checked and unchecked the relevant in FSUIPC and also as suggested adjusted the Thrust Limit in the FMC from CRZ to CON when things start to go wrong! It works the CON setting enables the aircraft to recover without to much of a problem.However, I also tried one other thing, ask ATC for a FL reduction, but nothing happens...why?I'm flying at FL 29,000 things go a little wrong and the ALT & SPD drop off...I catch this at FL 27,000 and SPD 0.650 and ATC saying over and over "climb to FL 29,000". I request a Cruise Altitude Decrease of -2,000 ft, but ATC doesn't even reply. ATC just keeps saying FL 29,000, but if I could climb I can get a Cruise Altitude Increase of +2,000 ft...why? why? why?Can anyone help me out..?Many thanks...Pablo Dominguez

[opherben 0]

Hello Darren, Excellent discussion! Some additional comments about your point quoted below. Climb in both aircraft types and watch the climb rate 1000 feet below OPT ALT. You will likely see similar results, which indicates similar specific excess thrust. I haven't tested before writing, but would estimate that rate of climb to be around 800ft/min. Aircraft weight is readily figured into that equation by determining OPT ALT per gross weight. The instance where weight plays a role in such substantial sudden airspeed loss, is the need to accelerate back to cruise Mach number. The way the PMDG model is designed currently, only 2-3%N1 excess thrust is available, which is likely an error, although engine efficient. Boeing would have likely designed 5-8%N1 as minimum, because the aircraft cruises right near the backside of the thrust/speed curve. You can see that drag characteristic by aircraft behavior after speed loss, and by noting that climb speed typically is cruise speed up high.>The amount of excess thrust available at high FL's is very>small at heavy weights on the 400 so supervision is required>just like in real life.The 763 has a fair amount of excess>thrust so doesnt suffer to the same degree as the 744.

[opherben 0]

Hello,it might be connected with FS9 ATC using older non-RVSM rules over North America.Why would you want to fly lower? stronger tailwaind is an advantage. Higher FL and Mach is always desirable. The PMDG B744 problem is not a function of wind intensity, but rather of drastic and non-realistic wind velocity change.>I'm flying at FL 29,000 things go a little wrong and the ALT &>SPD drop off...I catch this at FL 27,000 and SPD 0.650 and ATC>saying over and over "climb to FL 29,000". I request a Cruise>Altitude Decrease of -2,000 ft, but ATC doesn't even reply. >ATC just keeps saying FL 29,000, but if I could climb I can>get a Cruise Altitude Increase of +2,000 ft...why? why? >why?>>Can anyone help me out..?>>Many thanks...Pablo Dominguez>>

[Guest pablo_gsxr750_aus]

Thanks for the reply, but this raises a few ?'s and maybe you can help more..?Firstly, can you update the ATC Non-RVSM rules over NA?, and if so how?Secondly, I'm only trying to request a lower FL because the aircraft is going to take so long to recover to FL290 and obviously this would also shut that infernal ATC chat...climb and maintain FL290.Many thanks so far...Pablo DominguezPS - Another note on ATC is that whilst trying to fly KJFK to EGKK, ATC told me to turn left to 045 and resume my own nav just after take-off from KJFK, from here ATC left me alone bar the odd change freq until I reached Greenland N, NW of Icland, from here ATC instructed me to turn North for ever and a day. I in the end had to cancel the IFR, resume my own Nav to Gatwick and land...could this also be because of the Non-RVSM rules? My sid-stars are up to date for what this may help.Cheers, Pablo

[Guest Darren Howie]

HI OpherI would not be so sure about the excess thrust at crz power.On the A320 if we are climbing at max weight to say 36000' it will take at least 3 mins to go from .76 to around .79 level at 36000.The amount of excess thrust is small and the window for speed control small also.The 76 is a pretty highly powered aeroplane and imagine its pretty good.Any 400 drivers around to talk about the aeroplane accel ability at crz thrust but given it is only at crz rating wouldnt imagine it to be to high?Especially if you let the speed bleed back by 30-40 kts.Darren

[Guest]

Darren, what would you suggest I set the fsuipc wind restriction to? How many knots per second?

[Guest Darren Howie]

Hi AlI have mine set to a conservative value of 1 degree/kt per second.Stops any issues with the Queen and also the SST Concorde which gets a bit touchy near Mach2.Below 20K i change it back to around 3 kts per sec which is a little better for approaches.Darren