747 Take off Question

[paulyg123 135]

I love flying the PDMG 747-400. Tell me if this is a realistic part of the flight model:With 466,000 lbs ZFW and 80,000 lbs of fuel, I find the 747 goes from 0 to Vr in only 20 seconds. I always time the takeoffs on real planes and I get 30-40 seconds usually on a 757 or a 767. I am not redlining the engines either. So is this a flaw in the acceleration model, or can a 747 reach Vr in 20 seconds?

[scandinavian13 5,192]

Asking questions like this is likely to draw the ire of people like me, who will come to point out that numbers are never constant in aviation. What is 20 one day is 30 the next because of weights, weather, and company SOP.So, it depends. What's the weather? What's the temp? What's the pressure? Are you using a de-rate? Are you using assumed temp?Were those 757s and 767s?

[Matt2218 4]

What you are experiencing makes sense to me. Relatively low takeoff weight in the 744 vs. a probably well-loaded and derated 757 or 767 takeoff...

[paulyg123 135]

OK,I did leave out the important variables - I know, but the takeoffs I quote are from me as a passanger on a real 777, 747, 767 doing international flight on a packed plane. As a PDMG 747 pilot, I just notice at sea level, standard pressure, no winds, it seems that a 520,000 lb 747 takes off way too soon (20 secs) with flaps set at 10 degrees. As a passanger,I always time the takeoff from enginees spoolong up to wheels up - and I usually get 35-45 secs - never 20 secs in a jumbo.But I was on a JetBlue A319 at JFK 31L and we were wheels up before the threshold. We only needed the run up area to take off.

[jnr641802 6]

OK,I did leave out the important variables - I know, but the takeoffs I quote are from me as a passanger on a real 777, 747, 767 doing international flight on a packed plane. As a PDMG 747 pilot, I just notice at sea level, standard pressure, no winds, it seems that a 520,000 lb 747 takes off way too soon (20 secs) with flaps set at 10 degrees. As a passanger,I always time the takeoff from enginees spoolong up to wheels up - and I usually get 35-45 secs - never 20 secs in a jumbo.But I was on a JetBlue A319 at JFK 31L and we were wheels up before the threshold. We only needed the run up area to take off.

I used to time the take off run out of Nairobi northbound to London on the 747-200 back in the 1970's as it was always famously long winded.The longest run I recorded on a very warm evening was 1 minute 25 seconds from brake release(take off power already set) to lift off!!.It must have been rather "interesting" for the crew...regards Jim

[CaptainTim 0]

With the altitude at naorobi(sp) you need a long run up and with the temperature aswell then you need a long run up. Even in the MD11 it was a lengthy roll.In regards to the short take off time, is it your pushing the throttles all the way forward into the red? I have done many flights, usually at around the max ZFW for the flight with up to 130 tons of fuel on some routes. No derate allowed and we usually rotate within the last 3/4 of the runway, perhaps even 4/5ths. Thing is there are many different variables that will effect take off weight. Unfortunatly you are talking in lbs and I do not understand them so I can't comment on how heavy your aircraft is. The higher the temperature at the airfield, the higher the assumed altitude will be and subsequent performance. In short, the higher the outside temperature, the longer takeoff distance you will require to get airbourne and most likely the shallower the climb out will be. The colder it is the short distance you will need and most likely steeper climb out however this will depend on many factors. Its more of an idea into what happens overall.It might also sound like your not loading the aircraft properly. If you don't already, you will need to use the PMDG config for the 747 to load the fuel and passengers, I do this before everyflight as I have noticed the plane will revert to an almost empty load and the last amount of fuel that was in the tanks on my last landing.I hope this helps/give some insight into to the take off performance. Brief I know but I'm at work and being covert on AVSIM.Cheers,Tim

[HiFly 16]

To save fuel and excess wear and tear on engines, full takeoff thrust is seldom used on less than MTOW unless unusual conditions dictate (for example high density altitude, short runway, etc.). The information given by the original post indicates a very light TKOF wt and in real life that would require pilots to use a calculated reduced thrust commensurate with the weight, wind, density altiude, and available runway length. Today, with high fuel costs, this has become an increasingly important procedural requirement by all commercial carriers. The comparison used by the orignal post is meaningless. With engines powerful enough to lift a 400T fully loaded B744, using full thrust on such a light load would indeed accelerate to the low Vr required extremely rapidly. Usually the calculated thrust selected is designed to use up about 3/4 of the runway. The longer the runway and the lighter the load the less thrust would thus be required and the acceleration would be proportionately slower. I am a retired real world pilot and I have experimented using the de-rate formulas recommended by my former company and the PMDG performance has been remarkabley "right on" compared to my real world experience!Craig

[paulyg123 135]

Craig:That was the answer I was looking for. I just wanted to know how realistic was the acceleration of the PDMG 747 during takeoff. I guess that is how I should have phrased my question. You say it is right on - that's good for me. Thanks.

[Andrea1 6]

With 466000 lbs of ZFW and 80000 lbs of fuel if you perform a "full power takeoff" also considering a "static trusth" per engine of 58.000 lbs (according with pmdg 744 manual higher static thrust engine models are simulated also if in .cfg file is reported "58000 lbs") you got a total of 232000 lbs of trusth that means a weight/ttrusth ratio of 2,35 than is normal that you, if not at Denver about 6000 ft higher than the sea at maybe 30 °C of OAT, that you accelerate like a rocket. (it is explained also in the PMDG B744 takeoff lesson)..in fact in that case except if you had a 6000 ft long runways using full power is a non-sense...it serves only to add wear and tear to the engines....and the plane is more difficult to control during takeoff and initial climb...keep in mind that a 2,35 w/t ratiois equal to a Learjet 25 executive jet...also the Concorde, at about full takeoff weight (as usual for the Concorde) had a T/W ratio of 2,44...if I were you in that case (if no runway length or obstacles clearence are relevant) I would have used a "T.O. 2" setting...Best RegardsAndrea B.