I am trying to plan my next flight strictly by the numbers, but am having severe problems understanding the Required Takeoff Field Length table, which appears in the Takeoff Performance section of the PMDG AOM, 1-3. The stated purpose of the table is to enable one to determine the amount of runway required for takeoff, taking wind into account. However, I am finding it very difficult to interpret this table in a way that makes sense!For example, according to the table, if the available runway length is 3,900 feet, you will require 3,900 feet for takeoff in no wind. On the other hand, if the available runway length is 11,800 feet, you will require 11,800 feet for takeoff under the same conditions, and so on, for all the runway available lengths in the 0 wind column of the table!!! Now that cannot be right, can it?Surely, the column on the left should read REQUIRED Runway Length? The purpose of the table would then become clear - it is to adjust the required length for takeoff, taking wind into account. BUT...., if this is correct, it requires me to know my (uncorrected) required runway takeoff length before I use the table, which makes the table not very useful after all!!Somebody please explain where I've got this wrong, please!BR,Frank

Hi Frank,EDIT:Better answer below!JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.precisionmanuals.com/images/forum/ng_driver.jpg

Frank,The table in the PMDG AOM is a bit ambiguous. The table on page 1-3 has the right column titles but the main title "REQUIRED TAKEOFF FIELD LENGTH" is not really correct. There is also another error in this table. Negative numbers are for tail wind component and head wind components are positive. This table is for the -600 so let's do a calculation.This table is used to determine your field limiting weight and is to be used in conjunction with the next table on page 1-4 "Runway Length Limit Weight"Let's say you are taking of from a runway with an available length of 3,900 ft. You have a headwind component of 10 knots. According to the table and the above mentioned corrections the corrected runway length is 4,167 ft. Now take this corrected length and find the right row in the next table, page 1-4, We end up clos to the row for 4,200 feet corrected runway length and find the column that corresponds to the present outside air temperature. It's currently 18

Matts where did you get that dispatcher from?regrads Rob

Hi Matts,Very good answer, though you're being a bit generous when you state that the PMDG AOM is ambiguous. It has some pretty basic errors that I've been unsuccessful at getting them to fix or even admit to. I highlighted these errors back in May in the post reproduced below:The instructions for using the required takeoff field length and runway length limit weight in the PMDG 737NG takeoff performance documentation are incorrect.In the required takeoff field length table, positive wind values are for headwinds, while the negative wind values are for tailwinds. (As an aside: you will need special operational approval from the FAA if you want to take off in winds greater than 10 knots if operating under FAA authority.) This table is used to determine the wind corrected runway length to be used as an entry into the runway length limit weight table. For example, for a 737-600 operating on a 5,600 foot runway with a 20-knot headwind, the corrected field length would be 7,152 feet. With a 10-knot tailwind, the corrected field length would be 5,610 feet.To use the runway length limit weight table, step one is to determine the corrected field length from the required takeoff field length table as I've just described above. Enter the table with the corrected field length (NOT the runway length), then follow the instructions provided in the PMDG documentation. For the examples above, with a 20-knot headwind at 30 degrees C, the maximum field length limited takeoff weight would be 159,800 lb. With a 10-knot tailwind, the field length limit weight would be 150,860 lb (if I've done my math right -- using linear interpolation in the table). But since the climb limit weight is 149,500 lb, you would be limited to 149,500 lb in either case.Note that the tables are only good for one airport pressure altitude (presumably sea level) and one flap setting (not identified).Looking forward to your dispatcher program. Where did you get the data to use with it?Don S.

Hi Mats and Don,Thank you both for clear explanations of how to use the Required Takeoff Field Length Table, and the associated Runway Length Limit Weight table. I think I understand what you're saying(at work now, so cannot actually use the tables), but just to clarify the following for myself:"Let's say you are taking of from a runway with an available length of 3,900 ft. You have a headwind component of 10 knots. According to the table and the above mentioned corrections the corrected runway length is 4,167 ft."This means the headwind gives me, IN EFFECT, a longer runway for takeoff. Yes? This makes good sense to me(and is the reason why the takeoff runway in use will be the one facing into the wind). I just want to make sure it does NOT mean that I REQUIRE a runway of 4,167 ft for takeoff?(Probably still being confused by the title, huh?)Will go through your instructions in detail when I get home tonight.By the way, is it possible to get a complete set of tables for the NGs?Thanks again, guys, for taking the trouble!Rest assured I will have more questions later :-)BR,Frank

Hi again Frank,>This means the headwind gives me, IN EFFECT, a longer runway>for takeoff. Yes? This makes good sense to me(and is the>reason why the takeoff runway in use will be the one facing>into the wind). I just want to make sure it does NOT mean that>I REQUIRE a runway of 4,167 ft for takeoff?(Probably still>being confused by the title, huh?)Yes. That is correct. When calculating your runway limit weight the corrected runway length can be longer than the actual runway. But of course you still only have the available runway length to take-off from.The runway limit weight will ensure that you can safely take off on the available runway length with one engine out at V1 and reach 35 ft before the end of the TODA (Take-Off Distance Available or the actual runway). See below link for some runway terminology.http://www.aviationshop.com.au/avfacts/sample/perinst1.PDFIn other words. If your planned weight is the same as the field limit weight you will use the full runway to get to 35ft at the runway end with one engine out. The tables provided in the PMDG AOM seems to be relevant for airports at sea level and with a flaps 5 configuration.Too bad FS doesn't model runway slope as this also has a significant impact on the takeoff weight. Hope it helps,

Thanks, Mats, and also for an interesting PDF.The only thing that worries my aging brain is how complex all this is becoming(but I wouldn't have asked if I didn't want to know, now would I? :-)).Best wishes,FrankPS Is the Virtual Dispatcher your own creation? /F

"Note that the tables are only good for one airport pressure altitude (presumably sea level) and one flap setting (not identified)."I don't think pressure altitude need be factored in until you come to determine required takeoff thrust? This is done using the Takeoff Thrust N1 table. In other words, for a given runway length and aircraft weight, your thrust requirements will vary with the pressure altitude, and will come from the N1 table. What do you think, Don?Yes, for sure, the available runway lengths and corrected field lengths should be tied to a particular flap setting.By the way, I've discovered another small mistake in the tables - the runway lengths, 5,600 and 5,900 are in the wrong order in the left hand column of the Required Takeoff Field Length table(1-3), though I think the rest of each row is correct if you just switch the runway lengths.BR,Frank

Frank,The numbers in the tables are calculated using takeoff thrust. Therefore, they are tied to a particular pressure altitude.The purpose of the N1 table is to ensure that you, as the pilot, set the correct N1 for the conditions so that you get the takeoff thrust upon which your takeoff performance numbers were based.Don S.

>Hi again Frank,>>>This means the headwind gives me, IN EFFECT, a longer runway>>for takeoff. Yes? This makes good sense to me(and is the>>reason why the takeoff runway in use will be the one facing>>into the wind). I just want to make sure it does NOT mean>that>>I REQUIRE a runway of 4,167 ft for takeoff?(Probably still>>being confused by the title, huh?)>Yes Frank, you have it exactly correct. The effect of the headwind is exactly the same, performance-wise, as lengthening the runway. A pretty elegant way that performance engineers have come up with to account for the effect of wind in a simple tabular format, no?>The runway limit weight will ensure that you can safely take>off on the available runway length with one engine out at V1>and reach 35 ft before the end of the TODA (Take-Off Distance>Available or the actual runway). See below link for some>runway terminology.>A couple of minor technicalities --the engine failure is actually assumed to occur one second before V1. And if there is clearway present, up to half of the distance from liftoff to the 35 foot height can occur over the clearway. Also, the airplane must be able to complete a rejected takeoff from V1, either with or without an engine failure one second beforehand, and stop within the runway available (plus any stopway, if present) without the use of reverse thrust.Don S.

I'm confused again, Don. :-(According to PMDG, in the N1 table, "Resulting number is the approximate N1 percentage that will be ACHIEVED on a full power takeoff."(my emphasis).From what you say in your mail, though, it's the other way round. That is, you SET N1 rotation in order to get full power(in this scenario)?Here's my (mis?)understanding:"full power" refers to the position of the throttles, ie all the way forward. So, the table indicates the N1 percentage rotation that will be achieved(For example, at 41 deg F and pressure altitude O, pushing the throttles all the way forward will give you 90% N1 rotation.). Thus, the "full power" setting of the throttles produces VARIABLE amounts of thrust, dependent on temperature and pressure. The only question, then, is whether the amount of thrust produced is sufficient to get me off THIS runway at THIS weight(And how do I know that?).Don, as I may well have this all wrong, could you give some examples of what you mean, please?Thanks again,Frank

This post just to get my message above noticed(hope noone objects?:-)).BR,Frank

Frank,Before the advent of full authority digital electronic engine controls (FADECs), pilots would set power solely by reference to the engine power setting parameter. (For some engines, this is N1, for others it is EPR.) Pushing the thrust levers to the stops could result in an overboost. The takeoff power setting chart was necessary to ensure that the takeoff power used for the takeoff is the same as that used in determining airplane performanceWith a FADEC, the engine controller can match the full rated power at all temperature/altitude conditions with the thrust levers positioned at the stops. In this case, the takeoff power setting chart is used to confirm that proper takeoff power has been set by the FADEC.I'm on the road right now, so I can't confirm it but I'm 95% sure that pushing the 737NG's thrust levers to the stops will provide full rated power without resulting in an overboost. I'm not sure this is true of the PMDG, especially considering whether or not everyone's thrust controllers are properly calibrated. One way to check would be to see if a full throttle push would result in an N1 that matches the rating marker on the EICAS without going into the red band.Anyway, assuming that the 737NG engines are full throttle engines, pushing the thrust levers to the stops for a full rated takeoff should result in matching (approximately) the N1 numbers in the table.The bottom line is that the takeoff data are calculated for a certain thrust level -- takeoff, derate, or assumed termperature thrust reduction. The pilot must have a way to make sure that the same thrust is set for the takeoff as was used in the performance calculation, whether he does it by pushing the thrust levers to the stops, hitting the TO/GA button, selecting a derate or assumed temperature, etc. There must then be a way to confirm that proper power has been set by referring to a power setting table.Hope this helps,Don

WARNING. You are entering PMDG denial territory with this topic. Please take the next available taxiway and discuss something PMDG-friendly ;)Gary