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Guest Shaun48113

Problem with Manuals

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PMDG Team,I was reading the takeoff performance manuals tonight, and there seems to be a problem with the "required takeoff field length" charts for the -600 and -700. It could also just be me being dense, so please correct me if I'm wrong. :) I'll use the -600 chart as an example.It says that this chart is for calculating the length needed for a dry runway, full thrust takeoff. On the left is available runway length and across the top is wind component. Let's assume that I have a 15 knot headwind. Why do I need 2,789 feet of a 3,900 foot runway for a full thrust takeoff, but 13,845 feet of a 16,404 foot runway, again for a full thrust takeoff?I don't understand why the takeoff length is going up based on the AVAILABLE runway length if all other things are equal. Shouldn't I need 2,789 feet on either of these runways, wind and all other things being equal? It seems like the left column might have been intended to be gross weight or pressure altitude or something.-Shaun

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Hi Shaun,Good question. It's because the description of the table and the instructions for using it are incorrect. I posted the following a couple of days ago, but did not receive a response:"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 regards, Don S.

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