B737 Enroute Performance

B737 Performance chapter 2, Enroute Performance. By the end of this chapter you can explain what sets optimum versus maximum altitude, how cost index shapes cruise speed strategy, what actually moves top of descent, how drift down and its two strategies work, and why engine-out cruise can beat two-engine cruise on fuel mileage.

• 5 pages
• 66 min of reading
• 12 question chapter check
• Notes, highlights and progress saved

What this chapter covers

1. 1
   Optimum altitude and maximum altitudeWritten from Compiled from 737-800 performance principles and standard cruise planning practice
   13 min read
2. 2
   Cost index and cruise speed strategyWritten from Compiled from 737-800 performance principles and standard cruise planning practice
   14 min read
3. 3
   Descent planning and top of descentWritten from Compiled from 737-800 performance principles and standard cruise/descent planning practice
   13 min read
4. 4
   Drift down: obstacle clearance and fixed speed strategiesWritten from Compiled from 737-800 performance principles and standard drift down and ETOPS planning practice
   14 min read
5. 5
   Engine-out cruise and fuel flow characteristicsWritten from Compiled from 737-800 performance principles and standard cruise fuel planning practice
   12 min read

B737 enroute performance terms

Optimum altitude

The cruise level with the best fuel mileage, or lowest cost in ECON, for the current 737 weight.

Maximum altitude

The lowest of the 41,000 ft certified limit, the buffet limited altitude and the thrust limited altitude.

Buffet limited altitude

Where the manoeuvre margin to buffet runs out. The default margin is 1.3g, about a 39 degree bank.

Thrust limited altitude

Where the aircraft can just achieve a minimum climb rate, commonly 300 ft per minute. It falls in hot conditions.

Cost index (CI)

Ratio of time cost to fuel cost. CI 0 is maximum range cruise, CI 500 flies near VMO or MMO.

Long range cruise (LRC)

A fixed speed giving up 1 percent fuel mileage for about 3 to 5 percent more speed. Not wind corrected.

Maximum range cruise

The speed for the best fuel mileage, flown with cost index 0.

Step climb

Climbing in steps as fuel burns off to stay close to the rising optimum altitude.

Top of descent

Start of the idle descent. A headwind in descent moves it closer to the destination.

Driftdown

After an engine failure at cruise: maximum continuous thrust and the shallowest descent to a sustainable level.

ENG OUT CRZ page

FMC page giving the driftdown speed, engine-out maximum altitude and level-off altitude.

Net flight path

The driftdown profile with a safety margin applied, which must clear terrain by set amounts.

TSFC (thrust specific fuel consumption)

Fuel flow per unit of thrust. It improves at higher thrust settings.

Try a question from this chapter

B737 enroute performance questions