Perf Factors: Drag/F-F

[Will]

How are these used in real life?

1. How are they determined? (Dispatchers poring over old statistical data?)

2. Who inputs them? (Maintenance engineers?)

3. Are they checked or changed by the flight crew?

[Hardy Heinlin]

If the time is not accurate, I would first adjust the drag only, and keep the FF. When the drag is adjusted, the FMC will calculate a more accurate time which will also cause the FF to be more accurate. Then, if the FF is still not accurate, I would adjust the FF as well.


|-|ardy

[Will]

Thanks, Hardy. Am I correct in understanding that the entries on Instructor > Model > Airframe > Drag addition and Fuel Flow addition affect the actual model itself?

Like for example, if I put in a drag factor of +5% on the Instructor Page, will the actual aircraft fly 5% slower?

[Hardy Heinlin]

Yes.

[John H Watson]

 I'm not sure who compiles the FF/Drag values,  but Maintenance Control sends a list of amendments to the engineers once a month (with the nav database update disks).

2. Who inputs them? (Maintenance engineers?)

Affirmative.

3. Are they checked or changed by the flight crew?

They shouldn't be changed by pilots (but I'm sure many of them know how to do it). The values for the fleet are usually listed on a sheet in the Technical Logbook. I have heard of pilots picking up errors in entries: One month, someone decided to list the two numbers in reverse order, causing a few problems 

Rgds
JHW

[Will]

Thanks, John and Hardy. That's helpful.

[Will]

John,

Is it true that most aircraft will have some non-zero values? Or so most aircraft perform accurately without a need for any corrections?

What are the highest corrections you've seen applied?

Would pilots see a high correction and grumble about flying a "slow bird"?

[John H Watson]

My memory is a little fuzzy, but looking at a few 744 fleet Drag/FF sheets in my files, I see that some airlines don't bother computing Drag factors (and some don't bother computing Drag Factors for certain 747-400 types) i.e. they leave them at "0.0".

On these limited number of sheets, largest drag factor is "+1.6" and the largest FF value is "+5.0".

Rgds
JHW

[Hardy Heinlin]

Compare the precision of the drag/FF data to the precision of the forecast weather data. Imagine the wind/OAT forecast values had a tolerance of just 1.5 % ...

Imagine the weight & balance system had this precision, if there is one installed at all. Who knows the exact weight of the passengers?


|-|

[Will]

Good point, Hardy. I remember that airspeed tolerance on a flight exam might be +5 to -0 KTS on approach. Given an approach speed of 140 KTS, that's a tolerance of 3.5%. But some mechanical airspeed indicators aren't themselves necessarily that accurate.