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nocaltom

No length in fmc for departure runway

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I was flying kboi to kslc yesterday and started setting up flight in fmc for a departure on runway 10L. I selected 10L with no Sid but when I hit perf page I had no v speeds. I noticed it had no runway length listed. Same thing happened tonight on my second flight when departing from KDEN 34L no runway lenght listed in fmc. The flight before from Kphx to KDEN worked fine. What am I doing wrong?

 

Tom Davis

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Make sure that you have "QRH ON" on the right side under speeds, otherwise click the LSK next to it.

 

A thing to note:

FMC speeds are not calculated for individual runway. The FMC does not care for runway length, the speeds provided are for so-called "Balanced field length", that means, that for given aircraft wieght and enviromental variables, the distance to stop after V1 failure equals the distance to lift off after V1 failure. It is up to the pilot to confirm that balanced field length is less than (corrected) actual field length, and that the take-off is safe. There are tables for that in the QRH.

 

Normally though, pilots are given a runway analysis, either by way of book of tables, or by a way of computer program (or from ACARS), which will take more information into account leading to an unbalanced field analysis which may allow for better derate and/or more payload available. This takes into account things like clearway, which can be used for TO Distance comparison, but not for accelerate-stop distance nor take-off run (potentially leading to a higher V1go), stopway, which can be used for both accelerate-stop and take-off distance, but not for take-off run (leading to a higher V1stop, and possibly higher V1go) etc., things like Improved Climb procedure (leading to higher all V1, Vr, V2, more runway covered, but more climb)

If you are confused by more V1s mentioned, this is because actual used V1 is a set of several requirements, such as

V1 must be more than V1go, in that, V1go is minimal speed that allows for the airplane to safely take off after engine failure

V1 must be less than V1stop, in that, V1stop is maximal speed that allows for the airplane to safely stop after engine failure (this is the most commonly known V1 "definition" known amongst simmers, although it is incomplete, really - imagine V1 of 10 knots. You will stop in meters, but you can not legally stop after V1, so if engine failed at 11 knots, you must continue, and it would take immensely long for the one engine to propell the plane to Vr and V2)

Also, V1 must be more than Vmcg (minimum control ground - that means, you can control the airplane direction by rudder alone) and less than Vmbe (maximum brake energy - maximum speed, at which if you started braking on RTO, brakes, and tyres, can take it).

 

Note that this may leave you with a range of possible speeds, which is why you may see something like V1 120-125 in TOPCAT and alike. That means, you can stop safely at 125 and go safely at 120).

 

Then you have Vr and V2 that have their own set of requirements. Note this is all for field-limiting situation. If you are climb-limited, or obstacle-limited, there is more magic to play with, but generally it means moving V1 up to maximum limit, and playing with Vr and V2 to achieve optimal climb in second (and possibly later) segments.

 

(now I know I probably said too much for the question, but some people may find this interesting read, and hopefully a fruitful discussion will start)

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Pleased to help, feel free to add any questions if you want.

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