Ask Captain Lim ~ All about Aviation

Dear Sir,

As a Performance Engineer in Saudi Airlines, I have been asked by a pilot as to why they are using Flaps 15 instead of Flaps 5 where it gives, in most cases, higher take off weights.

Now I*m investigating and I will present a report and I need your help.

I have read what you wrote about best flap setting for the B777-200 Flaps 5/15 take off and you ended your answer by stating that there are other factors which can affect the choices too.

Best Regards,

Makrami

Hi Makrami,

Yes, during the take off on a Boeing 777, Flaps 5 is normally used. It gives the best lift and the least drag. That is why the 5 degrees flaps is also known as the *Take Off Flaps*. Of course, the captain (or the airlines may so recommend) has the liberty of using Flaps 5 and 15, depending on the circumstances of the case. Normally, one has to look at the take off weight, runway length and surface conditions. If the runway is short and airplane is heavy, then Flaps 15 is the choice.

For instance, the Narita Airport in Japan is a good illustration. It has a long runway ? 13,123 feet (16R/34L) and a shorter runway ? 7152 feet (l6L/34R). Say, if Saudi Airlines wishes to increase its frequency from 4 flights to 7 flights a week, then the airport authority may insist that the airlines use the shorter runway for the three additional flights (to ease congestion) or else they may not grant the extra flights there. In such a situation, Flaps 15 has to be the best flaps to use for the shorter runway.

Flaps 5 is used when the captain wants a good rate of climb after take off to clear some obstacles in the take off path; or where the runway concerned will allow a better take off weight - it would then require a further take off run ? meaning, a longer runway.

Many other factors would also affect the choice of the flaps settings. In fact, if windshear is forecasted during the take off, Boeing recommends pilots to use Flap 20 for optimum performance.

So, besides weight and runway length, other factors that can affect an airplane performance and choice of flaps are surface temperature, tailwind condition, runway surface (rough or uneven?), obstacles, altitude, runway slope (uphill or downhill?) and cost (accept shorter runway or else no extra flights to Tokyo - thus reduced revenue!)

So in general, Flaps 5 is normally used for take off on the Boeing 777 and Flaps 15/20 used on other situations where they warrant.