This is from page 18 of  CS 752 Flight Manual Part V The Flight Management System


SPEED/ALTITUDE CONSTRAINTS
VNAV controls the path and speed to comply with waypoint crossing constraints. Waypoint crossing
constraints are entered on the LEGS page waypoint line by pushing the applicable key on the right side of
the CDU. Values entered as part of a procedure and manually entered constraints are shown in large font.
FMC predicted values do not act as constraints, and are displayed in small font.
Waypoints can have altitude or airspeed/altitude constraints. Speed constraint entries require an altitude
constraint at the same waypoint. All speed constraints are considered by the FMC as at or below constraints.
At or above altitude constraints are entered with a suffix letter A (example: 220A). At or below altitude
constraints are entered with a suffix letter B (example: 240B). Mandatory altitude constraints are entered
without any suffix letter (example: 270).
Altitude constraints with two altitudes may be entered in either order. The lower altitude constraint, followed
by a suffix letter A, and the upper altitude constraint, followed by a suffix letter B (example: 220A240B or
240B220A).

Unfortunately the above procedure does not work. On a KJFK-KLAX  flight I used a RIIVR2 STAR as part of the arrival procedure. At the GRAMM waypoint the is a restriction to be at 280 kts below 21000 ft. and above 17000 ft. per FAA Chart ST237.

This image shows the entry in the scratchpad which will be RS to L5 and executed.


This image shows the CDU after execution. Who knows what the FMS is going to command the plane to do now. Rather than find out, I changed the altitude back to 21000 ft.


This image shows the HIS just after passing GRAMM. The altitude is fine and the speed bug dropped from 289 to 280 at GRAMM. The FAA Chart say to be at 280 kts. at GRAMM and the CDU shows the same 280 kts. at GRAMM. The 752 should not decide it’s going to slow to 280 once it is past GRAMM. This is hard restraint and should be met.


What happened next is for another post.

Michael Cubine

Out of curiosity, what happens if you enter the altitude constraint as /210B170A? Or were you just trying to see if putting them in with the lower altitude constraint first (/170A/200B)?

DAL191 wrote on Aug 20^th, 2013 at 7:16pm:


This is the next post. As can be seen from the next image the 752 has gone from VNAN to ALT HOLD and SPD. During the descent after GRAMM, VNAV reduced the speed to 240 kts. and maintained the vertical profile. All of a sudden at around 12000-13000 VNAV commanded a speed of 299 kts. and a descent rate of -4500 fpm. Approaching 10000 ft. VNAV changed to the mode you seen in the image.



So there is the 752 at 10000 ft. going to 299 kts. It is going to take awhile to reduce speed to 240 kts. and continue the descent to 24L.

Michael Cubine