Question regarding Required Takeoff Field Lengths

[Guest Henri W]

Hi,This is a question to PMDG. I apologize beforehand if this seems somewhat like a boring technical story, but for those that strive for realism and perfection it might be important.My question is related to the "Required takeoff field length" tables like the one on page 1-3 of the TAKEOFF section of the manual. I have the strong impression that this table means something different than what the explanation below the table describes. According to the description headwinds are indicated by a negative number and tailwinds by a positive number. I think it is the other way around. Why?1. The maximum allowed tailwind component for the B737NG is 15 kts, so why would you include a tailwind up to 40 kts and a headwind of only 15 kts.2. The titles of the colums do not make sense.Following the table explanation (when I understand it correctly), you should take the (required zero-wind dry-runway) takeoff length in the most left column, move to the column which corresponds to the head- or tailwind and read the actual takeoff length. (The header "AVAILABLE runway length" now really is confusing to me). So, lets take an example. Imagine my B737-600 needs a 7800 feet runway length for takeoff (in zero wind) and I have a 10 kts TAILwind, then, according to the above explanation, I would need 8169 feet takeoff length to compensate for the tailwind. See the figure below.http://forums.avsim.net/user_files/60393.jpgHowever, I think that this table might have to be read as follows:1. First of all tailwinds are indicated by a negative number. 2. The first column with header "available runway length" contains just that: the available runway length.3. The numbers in the table now refer to what the runway really corresponds to when correcting for tail- or headwind. E.g. a runway of 7800 feet length corresponds to a runway of 8169 feet (with zero wind) when there is a 10 kts HEADwind. In 10 kts TAILwind this runway only corresponds to a 6824 feet runway (with zero wind).Going back to my example of needing a 7800 feet runway length (in zero wind) while having 10 kts of tailwind, I read the table now such that I infer a required runway length somewhere between 8900 and 9000 feet. See the figure below (8500 feet only looks like 7415 feet, which is too little; 9200 feet looks like 8038 feet, which should be sufficient).http://forums.avsim.net/user_files/60394.jpgSo, while the manual tells you that you would need a runway of about 8200 feet (at least 8169 feet), I think you might need as much as 9000 feet, which is a significant difference.My question to PMDG or any other real-world pilot is whether my explanation makes sense.Looking forward to your reply.With best regardsHenri

[Guest rcarlson123]

I definitely see how this is confusing. Going by the paragraph describing how to use the table, it makes perfect sense that a headwind would reduce the takeoff roll. However, taken in the overall context, it seems that these tables are intended to be used to convert the true length of the runway to the actual length that should be counted on given the winds. In that case, a headwind should convert "available length" to a larger number.If the paragraph description is right, and these tables are to be used to calculate actual takeoff roll, then the table headers are certainly erroneous since we would not be working with "available runway length" or "wind corrected field length". Instead they should say "estimate zero wind takeoff roll" and "wind corrected takeoff roll".I know my reply hasn't answered your question, but I thought I would show that I share your confusion. :)

[Guest Henri W]

Thanks for reading my story and replying, Ross. I am glad I am not the only one that got confused. Lets see what the experts reply.Cheers,Henri

[Guest mkuebeler]

Yeah, I'm not sure I follow these tables, either. If the negative values are tailwinds it makes more sense to me. Say you have a 10,200' runway, but a 15 knot tailwind so the corrected runway length is 8,140. Now suppose it's middle of August in Texas with temps over 100 degrees F. The table on the next page would suggest that if your gross takeoff weight is over 148,000 lbs, you ain't gonna make it off the ground before running out of runway. The lower your weight, the less runway you'll need to get up to rotation speed, and as weight decreases for a given temp, so does corrected field length. With a headwind you can carry more weight given the longer corrected runway length. That's how I read it, anyway. Seems a bit harder to use than a table that gives takeoff distance required for a given weight. Also, I would think you'd need an adjustment for high pressure altitudes somewhere in there.

[Guest Henri W]

Yes, I read the tables exacltly as you describe: 148,000 lbs indeed is too much (be it in the case of single-engine failure, for normal operation you will rotate in time). I agree that there should be a correction for density altitudes.Best regards,Henri

[Guest Iz]

Yes Henri, you're correct. It should read (negative numbers indicate TAIL wind).

[Guest Henri W]

Thanks, Iz. I am wondering what PMDG says about it.Cheers,Henri

[Guest Henri W]

Just a quick bump to see whether somebody from PMDG can throw any light on this matter.

[Reset XPDR 1,271]

Bump again. PMDG, it would be nice if you could confirm that the manual is indeed incorrect in stating that negative values in the required takeoff field length table are for head winds. ie. that the positive values are really headwinds, thus increasing the 'equivalent' runway lengths for use in the runway length limit weight table.Gary

[Donstim 109]

Hi Gary,I had posted the following message on Dec 20th, but never received a response from PMDG:Kudos guys to your stellar update. I am continually amazed at what can be done within the framework of MSFS.Just a few comments regarding the performance data that you provided in the 737 PMDG flight manual documentation:1. In both the "Required Takeoff Field Length" and the "Required Runway Length Wind Correction Table ," the negative wind values are tailwinds, and the positive wind values are headwinds, not the inverse as is stated in the documentation. Contrary to what the documentation states, the resulting "Corrected Runway Length" is not the actual runway length that will be needed in those wind conditions. Rather, it is the equivalent "no wind" runway distance that would be needed. For example, a 3,900 foot runway, in the presence of a 15-knot tailwind, becomes an equivalent 2,789 foot runway in no wind. This reduces the maximum field-length-limited takeoff weight capability since the 15-knot tailwind effectively shortens the runway. (Note that the maximum allowable tailwind for takeoff is provided in the limitations, and is 15 knots, not 40.)2. The Runway Length Limit Weight tables for takeoff are good only for one flap configuration, one value of runway slope, and one altitude-temperature condition, but these values are not stated. Without knowing the values for the parameters used in deriving the data in the tables, one cannot realistically use this information to plan a takeoff. Also, without data for a range of conditions, one would be unable to plan takeoffs for other conditions. Any chance for providing a reasonable data set?3. The correct usage of the takeoff weight tables is as follows: a. Enter the "Required Takeoff Field Length" table with the available runway length for the runway being used for takeoff and the reported wind component in the direction of takeoff. (Note: you should probably provide a chart that would enable the user to determine the takeoff wind component from the total wind magnitude and direction.) A headwind is considered positive and a tailwind is considered negative. (Linear interpolation between columns can be used if necessary.)b. Read the Wind Corrected Field Length.c. Enter the "Runway Length Limit Weight" table with the "Corrected Field Length" from step b and Outside Air Temperature (OAT). (Note: "Wind Corrected Field Length" equals "Corrected Field Length" -- unless PMDG also plans to provide correction charts for runway slope, clearway, and stopway.)d. Read the Runway Limit Weight (interpolating between columns if necessary). This weight is the maximum takeoff weight for the available runway length.e. Enter the chart once again with the OAT. Read the associated Climb Limit Weight from the yellow highlighted row at the bottom of the chart. This weight is the maximum takeoff weight that will allow compliance with the climb performance requirements.f. Use the lower of the two numbers determined in steps d and e as the maximum takeoff weight. (Note that this weight must also be lower than the structural limit weight.)4. The instruction for what to do in case the runway is wet is not expressed in a manner that is particularly helpful for flight planning. It would be better expressed in terms of a weight decrement for a fixed runway length or as an equivalent reduction in the corrected runway length.5. The weight units in the wet runway takeoff speed table for the 737-600 are incorrectly labeled as being kilograms. The units are not labeled for the dry takeoff speeds for either the -600 or the -700, and the wet runway takeoff speeds table is missing for the -700.6. Any chance of providing landing data for Flaps 30?7. The procedure for determining the maximum field-length-limited landing weight should be similar to that described above for determining the maximum takeoff weight. You sort of have it backwards.8. Where did you get the information that the landing performance data "were acquired using a new aircraft with new brakes and tires?" This is not true for the 737NG -- the takeoff and landing performance data are based on brakes worn to their replacement or overhaul limit.These comments should not be construed as taking anything away from such a stellar product. I wouldn't expect you guys to be airplane performance engineers in addition to the many hats you already wear. But, if you're going to provide the information...Best, Don S.

[Guest Henri W]

Don,Thanks for this long explanation. I am really convinced now that a few things in the manual have been mixed up. Best regards,Henri

[Reset XPDR 1,271]

Don, Thanks for that. What you say makes sense.And now, a word from our sponsors .... (PMDG, this is your cue ;))Gary

[fozzy 1]

Hi All, i bring this topic up again just to check i got all which has been said.. :) Let's talk about the 700.. Required Takeoff Field Length (PAGE 1-8)If the runway is 5800 feet long and i have 20 knots headwind i can suppose the runway to be 6380 feet long for further calculations. (headwind as positive number not negative as stated) On wet runway I should consider the corrected figure - 5%? Runway Length Limit Weight (page 1-9)Using the figure from the previous step (6380)OAT 26

[Guest pagotan]

Hi all,With exception to the obvious lack of precision, logic and possible errors in the manual which have been mentioned in the previous posts, I believe that the problematic addressed is far more complex and beyond the scope of PMDG's conception of flight simulation. In other words PMDG is obviously in the business of developing aircrafts within the limitations of Microsoft flight simulators. To the best of my knowledge PMDG has not yet entered the business of flight despatching which what we are talking about here.This may explain the abscence of qualified respons from their developers. It is a pity that no simulation of pure flight despatching is available for the simmers community. FOC is as far as I know the first serious attempt but falls short of take off calculations. Lucky we are to have the outstanding FSBuild program but here again it will be of limited help for the problematic raised here. Having said that I would like ot offer some guidance on how it can be done nually.takeoff data calculation is a very complex theme, but here are some figures, how to work it out. MAIN PARAMETERS TO CONSIDEREngine types, OAT, obstacles, runway length, flap requirements, wet or dry runways, packs on or off, antiskid, break release weight, slope of runway, pressure altitude, winds, runway contamination.GENERAL On the basis of the rules of certification an aircraft is authorized to takeoff from any particular airport only if the takeoff weight is such to comply with several performance conditions. To determine performance data you need special charts provided in the Aircraft Operations Manual in the Performance chapter. AIRCRAFT CONFIGURATION Normal operation is with all brakes and antiskid operative for takeoff and landing. Corrections are shown on appropriate charts for antiskid inoperative. Corrections are also shown on appropriate charts for one or two brakes deactivated. The number of air conditioning packs = on is also noted on the charts with adjustments for alternate configurations. Wing anti-ice is off during all flaps extended operations. Corrections for nacelle anti-ice are shown on all effected charts. CHARTS YOU WILL NEED Takeoff field length requirements and climb performance vary with airplane gross weight, flap position, pressure altitude, temperature, runway slope, etc. With so many variables to consider, several charts are required to find the maximum allowable takeoff weight (MATOW). * Chart A - Runway Length Corrections This chart shows the effects of clearway and stopway, runway slope and wind on actual runway length. * Chart B - Maximum Takeoff Weight Field Length Limits These charts show the relationship between corrected runway length and takeoff weight as a function of airport pressure altitude and ambient temperature. Seperate charts are required for different flap settings. Line up distances are taken into account and corrections for engine airbleed are given on the appropriate chart. * Chart C - Maximum Takeoff Weight Climb Limits These charts allow the determination of limiting weights influencing the climb gradient of the aircraft in the takeoff flight path for obstacle clearance. Seperate charts are required for different flap positions. In addition, corrections for engine airbleed are given. * Chart D - Tire Speed Limit These charts allow the determination of tire speed limiting weights as a function of ambient temperature, airport pressure altitude and wind component. Seperate charts are required for different flap positions. * Speed tables These tables present takeoff speeds V1, VR, V2 and 3 engine climb attitude as a function of takeoff flaps, grossweight and atmospheric conditions. These charts also indicate performance areas affected by Ground Minimum Control Speed (VMCG). DETERMINATION OF MATOW AND V-SPEEDS Collect and fill in your T/O data sheet with the following data: - Airport - Runway - Runway Conditions - Wind data - Outside Air Temperature, OAT - QNH - Pressure Altitude, PA - Runway Length - Runway Slope - Number of A/C Packs for T/O - NAC A/I ON/OFF Now starts a calculation procedure to determine MATOW, V1, VR and V2 STEP 1: Determine the corrected runway length from chart A. STEP 2: Enter the appropriate chart B with the corrected runway length from step 1 and determine field length limited takeoff weight (TOW-F). STEP 3: Enter the appropriate chart C and determine second segment flight path limited takeoff weight (TOW-S). STEP 4: Enter the appropriate chart D and determine tire speed limit takeoff weight (TOW-T). STEP 5: Compare takeoff weights obtained in step 2, 3, and 4 with structural limit and use the lowest value as MATOW. STEP 6: Read V1, VR and V2 for actual TOW from speed tables. STEP 7: If takeoff speeds from step 6 are determined from the VMCG-limit area of the speed table, determine the lowest TOW above this area for respective OAT and pressure altitude from speed tables. STEP 8: If step 7 is effective, the takeoff is permissible only, if TOW-F from step 2 is greater or equal than TOW from step 7. To make life easier, airline crews use special Runway Weight Charts (RWC) provided for all reasonable runway directions of the regular destination airports and the major runways of the alternate airport to determine the performance limited takeoff weights (PTOW) for dry and wet runways using flap 10 or 20. Additional RWCs for contaminated runway are published. Corrections are provided for QNH, A/C and Nac A/I variations. Software like Aircraft Performance Modules are offered for today's airlines which allow staff to calculate all relevant Take-off Data on the spot. These programs are ideal for on-board use, on a notebook. A large part of this explanation was posted on another forum ages ago however not in relation with PMDG's aircrafts. Unfortunately no developer has shown interest sofar for a software which could make the serious simmers' life easier. As you may have noticed, if you did not get bored reading the above, Chart B is the key to determine the runway length. I believe you can get those charts for your preferred airports fairly easily by contacting the airports directly and then use them to compute your datas.I apologize to the readers for the length of this post but this was the most concise way I could get into some details.DISCLAIMER:The above reflects only a personal opinion and was written without any intend to offend, harm or criticize in any way or manner any individual, institution, association or company including PMDG.Michael

[Guest Darren Howie]

G'day HenriYou are 100% correct in that the manual is incorrect and should be the other way around,well spotted.EnjoyDarren Howie