B777 Flaps setting for dispatch landing distance calculations at alternate airport
The following question concerns the B777 flaps setting which is to be considered for dispatch landing distance calculations at the alternate airport, based on the expected landing weight at the alternate airport and therefore on the expected quantity of fuel on board.
It is a matter of regulatory planning for pre-departure and interpretation of regulations, not of in-flight fuel management and what may happen during the flight or what could be the crew options once airborne. This only concerns what it is we have to do before departure in terms of factored required landing distance calculations at the alternate airport.
In summary, calculations of the factored landing distance (60%, 1,15, etc.) are mandatory at dispatch for destination and alternate(s) airports. To perform these calculations we absolutely need a landing weight that is obviously based on the ZFW + remaining fuel. Again not talking about in-flight management, but only planning the flight and interpreting regulations, departing with no extra fuel means landing at alternate airport with only the final reserve fuel on board.
On B772 or B773, the final reserve fuel is less than 4T, thus landing with less than 2T in each main tank. This will trigger the "Fuel Low Qty" non normal C/L which calls for flaps 20 landing...
Dispatch landing distance calculations, however, are based on demonstrated/certified performances/capabilities available only for flaps 25 and 30, the OPT also only displays flaps 25 and 30.
Therefore, what is the flaps setting for the required dispatch (factored) landing distance calculations at alternate airport?...
I know what my airline is doing and why, but for the sake of reflection, I am very interested in knowing what others are doing, how and if it is for the same reason.
Thank's!
Now for the details:
As per IR OPS CAT.POL.A.105 "General" (c) "Due account shall be taken of aeroplane configuration, environmental conditions and the operation of systems that have an adverse effect on performance."
This CAT.POL.A.105 concerns "the mass of the aeroplane: (1) at the start of the take-off; or (2) in the event of in-flight replanning". This is clearly a pre-flight regulatory plan.
CAT.POL.A.225, 230 & 235 all refer to CAT.POL.A.105 for dispatch landing distance calculations at destination and alternate(s).
Fuel regulatory policy is also a matter of pre-flight planning.
CAT.OP.MPA.150 states that "The operator shall ensure that the pre-flight calculation of usable fuel required for a flight includes: taxi fuel; trip fuel; reserve fuel consisting of: contingency fuel; alternate fuel, if a destination alternate aerodrome is required; final reserve fuel; and additional fuel, if required by the type of operation; and extra fuel if required by the commander."
AMC1.CAT.OP.MPA.150(b) gives details of all these quantities, the alternate fuel being calculated for a missed approach from DA at destination to landing at the alternate.
Therefore, without extra fuel required and with contingency fuel being potentially used during the flight, the entire fuel policy regulatory process is based on landing at the alternate airport with only the final reserve fuel.
A crew can and will takeoff with this minimum regulatory fuel for saving the payload (ie Asia-Europe flights with a B773 at MTOW are fairly common).
The crew must verify that the minimum quantity of regulatory fuel is at least on board at the start of the take-off.
During the pre-departure checks, the crew also had to calculate the destination and alternate(s) landing distances with the pre-flight (dispatch) requirements (ie 60% dry factor).
The entire regulatory process "at the start of the take-off" is therefore based on minimum regulatory fuel and the obligation to also comply with dispatch landing distances accounting for "aeroplane configuration, environmental conditions and the operation of systems that have an adverse effect on performance" as per CAT.POL.A.105 c).
Whether the crew then decides to divert and land en-route, or anticipate the diversion to the destination’s alternate without flying down to the (M)DA/H, it is a matter of in-flight fuel management and not of regulatory planning as it was prior to the start of the take-off. In fact, once airborne, the crew will also only have to match the LDA without the factors of 60% or 1,15 and it won’t be based, at least for the B777, on the same landing data as it won’t use the certified landing distances but some more operational values.
Therefore, matching the fuel policy rules with dispatch landing distance calculations rules, as the IR-OPS is written, leads to a calculation of landing distance at the alternate airport on the basis of a landing weight equal to ZFW + final reserve fuel.
(One might wonder if the contingency fuel is assumed to be still on board. This will not be important on a relatively short trip departing with minimum regulatory fuel, and this contingency fuel is anyway meant to be used for unpredictable deviations of an individual aeroplane from the expected fuel consumption data, deviations from forecast meteorological conditions and deviations from planned routings and/or cruising levels/altitudes.)
The fuel quantity on board at the alternate airport will be the final reserve fuel which is about 3.1 tons for a B773.
With less than 2 tons in a main tank, therefore less than 4 tons of fuel on board, the non normal checklist "Fuel Low Qty" will display and call for a flaps 20 landing.
Knowing from dispatch that if you have the "aeroplane configuration" flaps 20 at the alternate it will thus have "adverse effect on performance", obviously increasing landing distance compared to a flaps 30 landing, do you take flaps 20 or flaps 25/30 for alternate airport dispatch landing distance calculations and why?
Do flaps 20 have to be taken in consideration because it is the only flaps setting that corresponds to the planned fuel remaining on board without voluntarily deviating from the FCOM/FCTM/QRH?
If you're thinking of considering flaps 20, how do you do that since we don’t have dispatch datas for flaps 20 landing?… Dispatch landing distance calculations are based on demonstrated/certified performances/capabilities only available for flaps 25 and 30, and the OPT only displays flaps 25 and 30...