Yup! It is all down to specifics but I have just had the chore of looking at a 737-800. So you are all going to suffer.
It is accepted that the take-off performance of an aircraft is directly proportional to the square of its weight and inversely proportional to the product of air density, wing area, lift coefficient and effective thrust.
I now refer you to the following URL on the official Boeing site:
http://www.boeing.com/assocproducts/...s/737wsec3.pdf
which is Fig 3.3.36 JAR Take-off Runway Length Reqmts. For a temperature of 30deg C and a wet smooth surface the performance of the B737-800 with CFM56-7B27B1 engines at 27,300lb static thrust is tabulated. At this point I must stress that in practice either or both the Boeing Laptop Tool and the on-board Flight Management Computer Systems are used to calculate exact settings. The generalised corrections implicit in paper tables err on the conservative side by as much as 5 tonnes and maybe double that on the 777.
I now refer you to the Quick Reference Handbook (QRH) for that very same bobby 737 aircraft and extract the following sea-level table.
Weight Weight V1 VR V2
tonnes Lbs kts kts kts
65 143325 140 144 150
60 132300 134 138 145
55 121275 128 130 140
50 110250 122 124 135
45 99225 115 117 130
40 88200 108 110 125
Back to school and develop from these parameters the implicit stall characteristics and lift coefficients eg an XL spreadsheet helps.
I have adopted here a particular day when barometric pressure and relative humidity are such that the airfield density altitude is 2000ft. There is now sufficient data to hand, albeit with generalised rather than specific corrections to predict take-off lengths and correlate them with the Boeing paper graphs using line regression techniques. A simpler analysis whereby JAR Length = 16.5(W^2/ Density ratio x Sw x CLmax x
[email protected]) + 500 provides almost indistinguishable results, specifically of course for the 737-800 and no other.
If you want a 744 or a 772 analysis (groan) I can do it but my figures will always underestimate the computer in weight capacity by anything from 3-7%.
Anyway notwithstanding all these gripes if I were a pilot in a B737-800 on the runway at FACT with Outside Air Temperature 30o C, Dew Point 15o C and barometric pressure 1017mb the JAR take-off length for 145,000lb gross weight would be 5995ft. Allowing 92,000lb weight for the aircraft operationally ready but empty and 115,000 for the zero fuel weight this would accommodate 100 passengers. My own spreadsheet tabulations are now shown.
Weight W2/sSw CLmaxT JAR Length
lbs Feet
145000 333 5995
140000 309 5599
135000 286 5219
130000 264 4856
125000 243 4510
120000 223 4180
115000 204 3866
110000 185 3553
105000 168 3272
100000 152 3008
95000 137 2761
I now refer you to the following site:
http://www.boeing.com/assocproducts/...s/737wsec3.pdf
and specifically Figure 3.2.2 which is a Payload/Range tabulation. You will see that subject to the alternate diversion, fuel reserves and flight profiles mentioned that the aircraft has a range of 2000 nautical miles. This is an extra margin of 300 nautical miles over my intended flight showing just how coarse paper charts with their generalised conditions can be.
What you really do of course is use the electronic hardware fitted and authorised but if your responsibility stops, so do the bucks in the paycheck and you have the honour to be first at the scene of the accident. So you do have to know how to use first principles to test and see if a localised issue makes sense.
Sorry if this is dreadfully long but as the enicalyth was just in the middle of having its patience tried by a complete drongo I am offering the paperwork with the expletives deleted. Now let me get out from behind this desk and go fly something, a kite even!
Once the whole shebang is certified, rubber stamped and the cat's mother has pee'd all over it I write the SOPs and you, Lucky Jim, do what the book says. So how long was that runway? Gawd, they've all nodded off!