landing distance calculations

[STS-41B]

YV pilot, if that matters in this scenario.
We fly to some short runways, Santa Barbara and Burbank among others. An ASAP report from last September got my attention: a crew diverted from landing at SBA due to a “spoileron fail” caution message. The QRH directs us to “add 20% to the actual landing distance.” The crew went to LAX, refueled , cleared the issue, went to SBA.. but not after dispatch argued with Captain that they could’ve gotten into SBA and they needessly diverted.

SO.. most of the time the runways are plenty long, so it’s not an issue. But sometimes it can be.
When we sent our ACARS message with our landing weight and conditions, etc... we get back the runway length, the “minimum required distance” and the “unfactored distance.” .. for example, min req’d distance might be 4800’ and unfactored 3003’. .. my understanding is if we have some issue that requires adding 20% .. we add to the 4800’. We’d need 5760’ now. Burbank is 5801 I think.. so we’d be good, right???
What’s throwing me is the verbiage of the QRH saying “actual landing distance” since our speedcards have actual landing distance listed and it’s nearer the smaller number we get on ACARS.. and my understanding is that smaller number is based on max braking with no wind and nailing the touchdown on speed perfectly.
And for fun let’s throw in a quick rainstorm and now the runway is wet. Min req’d would likely be ~6000.. unfactored still 3003. So w/a similar failure, we’d add the 20% to 6000 or whatever it is, right??
Maybe I'm overthinking it but i want to be confident in my knowledge and understanding of this...

 

[Deleted member 27505]

Sounds like you need Cessnav. Lol.

More seriously,
Sections §§121.195, 135.385, and 91.1037 prohibit the takeoff of a transport category airplane unless its weight on arrival, allowing for normal consumption of fuel and oil in flight, will allow a full stop landing at the intended destination airport within 60 percent of the Effective Runway Length.
Three Considerations:
–Aircraft is landed on the most favorable runway in still air, and
–Aircraft is landed on the most suitable runway considering probable wind, landing aids and terrain.
–If the runway is forecast to be wet or slippery at time arrival, the required field length is increased 15%.

AFM Actual Landing Distance Factors (Baseline Distance Calculated Based On...)
•Vref at 50’
•Cross threshold at 50’
•Approach angle between 2.5º & 3.5º
•Dry runway
•Zero slope
•Touchdown 1,000’ from threshold
•Touchdown sink rate 2-6 fps (maximum 8 fps)
•Standard Temp (TAS)
•Maximum manual braking
•No credit for thrust reversers

After saying all that, if anything is off significantly, or a stack of things are off slightly, you can still go off the opposite end. (e.g., a wet runway will likely add at least an additional 500ft per 10kts, floating will typically give you an extra 2500ft per 10kts.)

If my little voice is whispering, "this doesn't feel right," I do my best to listen to it.

 

[Autothrust Blue]

The argument is already over insofar as the Captain made an in situ decision concerning the safe operation of the airplane.

While the dispatcher is “technically” correct that the absolute performance capability - which is what INFLIGHT and EN ROUTE calculations use, not dispatch (with the fudge factor*) calculations - he’s still wrong insofar as he was not there and was not in the pointy end. I would be very hesitant to take a heavy CR9 into SBA with any sort of issue that concerned the stopping of the airplane, the tables be damned.

I landed one day in Madison, WI with a spoiler fault in a -9 and was sort of amazed at how adversely the stopping performance - the real performance - was impacted.

 

[bimmerphile]

FAA TV: Landing Distance Assessment