Sometimes a STAR will have an altitude constraint which requires an early descent and subsequent level portion.

 

In order to fly level for a time and ignore the geometric descent calculated by the FMC, the FCOM states that a new TOD can be calculated by entering the aircraft's current altitude on the CRZ page. For some reason, I can't get this to work.

 

Senario:

 

Let's say the STAR has restriction of FL150 at waypoint A. and 2000ft at Waypoint B. After reaching FL150 and passing waypoint A, rather than descend at 300fpm to waypoing B, you would like to maintain FL150 and re-commence the descent for an idle profile to waypoing B. When entering 150 as a new cruise altitude on the CRZ page, the entry is accepted, but as soon as I press EXEC, it jumps back to the descent page and the descent path indicator reverts to the geometric descent.

 

Any suggestions?

 

Many thanks,

This is what I do, I either;

 

1. Exit VNAV and descend in V/S or FL CHG down to FL150. While descending I punch in FL150 in the now boxed CRZ page. A new TOD will be calculated somewhere between point A and B. While descending I make sure to maintain or switch to the new cruise speed for FL150 (either the one the FMC Cost Index calculates for me or a manually entered one in the CRZ page). When I level off at FL150 I reengage VNAV.

 

2. Let VNAV descend down to FL150. Make sure 15000 is set in the MCP and not a final descent altitude for the STAR. After VNAV reaches FL150, punch in FL150 as your new cruise altitude in the CRZ page. It "should" recalculate your TOD.

 

3. The one that separates the men from the boys. Press altitude hold prior to TOD. Calculate your TOD (altitude difference/1000x3) compensate for descent speed, winds aloft and actual weight. On most airplanes this formula is for a descend speed of around 300-320 KIAS and average landing weights. In other words if I am at FL350, my TOD will be at 35000-15000 / 1000 x 3 =60 NM from point A. I will be near max landing weight so I add 10 NM. I have a headwind of 10 knots during my descent. I will be descending for about 8 minutes to reach 15000, so I add (8 min=0.13hrs@10 nm/hour headwind is 1.3 nm. My TOD corrected for weight, wind and altitude loss is: 60+10-1.3=68.7 NM from point A.

 

Point 3 really tests your airmanship. I don't suggest you do this one just yet. But try making some calculations and doublecheck them against the VNAV calculated TOD. You'll get to know the NG better. And then one time when there's no ATC online, you can put point 3 to the test.

 

Cheers !

While not exactly an answer to the OP's question, an interesting thread on PPRuNe I found might shed some light on the underlying FMC mechanisms at work here: http://www.pprune.org/tech-log/382263-737ng-vnav-question-geometric-idle-descent.html. Cheers.

This is what I do, I either;

 

1. Exit VNAV and descend in V/S or FL CHG down to FL150. While descending I punch in FL150 in the now boxed CRZ page. A new TOD will be calculated somewhere between point A and B. While descending I make sure to maintain or switch to the new cruise speed for FL150 (either the one the FMC Cost Index calculates for me or a manually entered one in the CRZ page). When I level off at FL150 I reengage VNAV.

 

2. Let VNAV descend down to FL150. Make sure 15000 is set in the MCP and not a final descent altitude for the STAR. After VNAV reaches FL150, punch in FL150 as your new cruise altitude in the CRZ page. It "should" recalculate your TOD.

 

3. The one that separates the men from the boys. Press altitude hold prior to TOD. Calculate your TOD (altitude difference/1000x3) compensate for descent speed, winds aloft and actual weight. On most airplanes this formula is for a descend speed of around 300-320 KIAS and average landing weights. In other words if I am at FL350, my TOD will be at 35000-15000 / 1000 x 3 =60 NM from point A. I will be near max landing weight so I add 10 NM. I have a headwind of 10 knots during my descent. I will be descending for about 8 minutes to reach 15000, so I add (8 min=0.13hrs@10 nm/hour headwind is 1.3 nm. My TOD corrected for weight, wind and altitude loss is: 60+10-1.3=68.7 NM from point A.

 

Point 3 really tests your airmanship. I don't suggest you do this one just yet. But try making some calculations and doublecheck them against the VNAV calculated TOD. You'll get to know the NG better. And then one time when there's no ATC online, you can put point 3 to the test.

 

Cheers !

Press the MTRS button, that tells you your miles to go on a 3 degree descent path to sea level