Hello!

So some flights I have been doing lately (longer ones) in the 737NGX, I have been putting in higher and higher altitudes in the FMC. But at the T/D, the plane would go between -1200 to -2400fpm, then ~10nm from my selext alt, it would tell me "Dest. Path unachievable". I usually over ride this by manually putting in the -vs in the MCP. That would result me in going 300kts under 10,000.

Anyone know a way where I can fix this? On shorter flights, this isn't a problem, but it seems that the distance between T/D to select alt. (usually between 3800 and 1800) is manageable for RNAV>

 

Thanks!

-Valley.

Make sure you're at idle. What's your N1 reading?

Watch this, all the way through. Study it and take notes.

http://www.youtube.com/watch?v=4MrFpbh7Pq8‎

 

http://www.youtube.com/watch?v=4MrFpbh7Pq8‎

Kopskip. Thank you very much for the video! I am going to need to watch it multiple times to get the lesson.

Okay. I know what's up. When it comes to the Cost Index and Reserves, I put in random numbers.   How do I calculate the PROPER information to go into the boxes?

Plug in some enroute winds and most importantly forecast descent winds so the fmc knows accurately how heavy you'll be(extra or less fuel bc of winds). Winds during descent will affect the fmc's ability to stay on profile. Often all you'll need to get back on path is to speed intervene and descend a little faster. V speed is not what you want to use. Flch or vnav speed intervene is what you want. CI will have little do with your descent being achievable (if the number us reasonable) and reserve even less. It only serves as an alert.

Cost index is a function of accountants. The lower the cost index, the slower the plane flies in climb/descent and to an extent cruise too. The higher the cost index, the faster it flies.

 

Some airlines assign a different cost index to every flight, some have 2 standard cost indexes ('on time' and 'late') that they use all the time. Cost index shouldn't cause the VNAV to come in high, but a high cost index without an accurate descent winds speed profile inserted into the Descent forecast page can be problematic.

 

If you are flying a short flight where you don't get above FL290, you may find a higher cost index in use due to the cruise mach number being below 0.65 in some profiles that low.

 

Reserves is... harder to explain. 

There's the actual reserve fuel you will be arriving at the airport with, and a lower number for the reserves when you will need to take the decision to divert if you reach that number.

 

ie: Usually I arrive at an airport with between 2.3 and 4.0 (metric) tonnes of fuel left, but the fuel critical is Diversion fuel + the 45 minute reserve or 60 minute reserve depending on the carrier's country of origin's rules and a few other things. This means the number placed in the FMC for "Reserves" is usually near 1.9 metric tonnes, as much as 2.9 depending on how far away the alternate is and/or even if an alternate is carried (there are some places that, depending on weather forecast, may not require an alternate. Australia is one... sometimes.)

 

Also make sure your path is one that flows right through to approach. These approaches that have a Vectors section will never let LNAV be used all the way through to glideslope capture, and you will need to use FLCH or similar modes instead.

So both of you mentioned a FLCH mode. Where is that in the cockpit relative to the MCP? And would I be able to get the specific Cost INDX and Reserve for my flight under the fuel page in the FMC?

LVCH= LVL CHANGE I just noticed that. XD

LVL CHG is the 737 analog to FLCH in the other Boeings.

Would I find the Cost index and reserve  for my specific flight under the fuel page in the FMC?

Init Ref button on the FMC (then select menu)

"PERF" page.

It doesn't tell you what the cost index and reserve is, you tell it. But there will be spots where you put the numbers in.

 

I usually use a cost index that varies with steps of around 15, 22, 30 & 65, depending on how high I'm flying. The lower you fly, the higher the cost index. Some specific airlines use specific different variables. Lower number = costs less but gets there slower, Higher number = burns more fuel but goes faster.

Would I find the Cost index and reserve  for my specific flight under the fuel page in the FMC?

 

The cost index is something your airline would give to you. It depends on a lot of variables.

For example, some airlines pay crew for actual time flown, others for scheduled time flown.

The first ones will have higher cost index, the others lower cost index. It depends on fuel price and flight time based costs. So it changes from flight to flight. Some aiports have cheaper fuel, other more expensive fuel. Sometime you would fly with 3 F/As, sometimes you need 5, depending on route. (in a 700)

 

 

Watch this, all the way through. Study it and take notes.

 

haha - I was just about to go over to YouTube to grab a link to that.  Thanks for the link and support!

 

For further information on how to handle all of this, here's somewhat of a follow up video about descent forecasting:

In regards to forecast descent winds everyone has their own technique. Most of the guys I fly with don't bother entering anything if you have a headwind. If you have a tailwind you always want to enter something. In the tailwind case most guys will enter the actual wind for the altitude you are cruising at, 10000 feet below cruise and 20000 feet below cruise using the same winds but subtracting 20 knots each time. Others will put in the exact winds at those altitudes. I've tried both and the end result is pretty much the same. Some will double a tailwind and enter that. I'll always put in the altimeter setting and anti-ice. No matter how accurate your forecast is some intervention is usually required when descending in VNAV.

Of course I'm not in an airline right now, so is there abase line/default Cost Indx and Reserve for FL 200, FL300, and FL 400?

Of course I'm not in an airline right now, so is there abase line/default Cost Indx and Reserve for FL 200, FL300, and FL 400?

Here's a good start for you.

http://forum.avsim.net/topic/336375-cost-index-database/?p=1986979

 

Cheers,

Jeff