B737 T/O Performance - V SPEEDS UNAVAILABLE

[STBYRUD]

Hello gents,
ran into a problem recently with the trusty 737... Departure from Nairobi, runway 06, slight tailwind of four knots. B738 SFP, heavy at 78.5 tons estimated TOW, 16°C. According to the T/O performance charts the best bet was a Flaps 1, no bleed, improved climb takeoff. V-speeds in the charts obviously quite high. Entering flaps 1 on the Takeoff ref page blanks the QRH speeds with the 'V SPEEDS UNAVAILABLE' message - so the FMC would exceed the maximum V speed increase to stay above minimum control speeds... The FCOM says 'takeoff is not permitted' - according to the takeoff performance charts allow for a takeoff in this case though, with fitting high V-speeds... Eventually the TOW was nearly two tonnes less on the actual loadsheet, we ended up doing a flaps five takeoff (no bleed again) - the FMC happily provided v-speed suggestions for flaps five.

In any case - if the takeoff performance charts allow for a takeoff (giving own V-speeds) and the FMC gives 'V SPEEDS UNAVAILABLE' - where do we stand? The speeds from the takeoff analysis charts will allow for enough margin to the control speeds, so can that message be disregarded? Any ideas? Reported this to the performance department as well, I will post their answer...

[Denti]

The QRH speeds provided from the FMC, where that option has been selected, only very old planes have it in our fleet, are advisory only in any case and the speeds from the performance calculation will be used. If a take off is not possible no speeds will be given out by the performance program. To be honest i just use the QRH speeds as a very rough cross check if at all, switching them off declutters the T/O ref page 1 considerably.

[germanflaps]

T/O performance charts you say? As used in smaller aircraft, e.g. ATR or Dash-8? I probably wouldn't be comfortable using them on the B737, much less when it comes to maximum performance. We use a comprehensive program on a tablet PC that provides us not only with the speeds and de-rates possible but also shows the limiting factors (i.e. rwy length, required climb performance, etc.). QRH speed references are usually off by a few knots, but can make you notice false entries on either end when the difference is too big - a good way to cross check your speeds. :)

[de facto]

To be honest i just use the QRH speeds as a very rough cross check if at all, switching them off declutters the T/O ref page 1 considerably.

Same here ,only use when data from the RTOW are not available.

The speeds from the takeoff analysis charts will allow for enough margin to the control speeds, so can that message be disregarded

I would have disregarded it and entered the data from your chart,however it is a good idea to notify your airline so they can inform crews that may run into the same issue and decide to offload luggages:E

[Denti]

Performance charts might mean the older style RTOW tables in a quite thick book. Used those for many years on the 737 and i suspect i might be in the same or a similar outfit like you are germanflaps :E

Nothing wrong with those, it is usually much faster than filling in all the stuff on the tablet pc, however forgetting to change the registration or runway and flying off with the wrong performance. A quite common occurrence sadly enough.