Fix a missing PMDG FMC descent path by checking the active route, PERF INIT, discontinuities, constraints, EXEC status, VNAV and MCP altitude.
PMDG FMC descent paths are usually missing because the active route is incomplete, PERF INIT lacks a valid cruise altitude or weight, an arrival change is unexecuted, or a discontinuity breaks the leg sequence. Complete and execute the route, correct performance data and constraints, then make sure the MCP altitude is not blocking VNAV descent.
In PMDG Boeing aircraft, a missing calculated path and an aeroplane that refuses to descend are different problems. Exact CDU labels vary between aircraft and simulator versions, but the route, performance and mode logic is broadly the same.
What does the missing indication mean?
A missing top-of-descent symbol, a level aeroplane at top of descent and a path warning point to different faults.
| What you see | Likely cause | First action |
|---|---|---|
No T/D symbol or descent prediction | Incomplete route, missing performance data, broken leg sequence or unexecuted modification | Check the active route and PERF INIT |
T/D exists but the aircraft stays level | MCP altitude has not been lowered, or VNAV is not the active vertical mode | Set the cleared altitude and read the FMA |
DES PATH UNACHIEVABLE or DRAG REQUIRED | A path exists, but the aircraft is too high or fast to follow it | Correct speed, drag or an erroneous constraint |
The T/D may simply be beyond the selected Navigation Display range. Check the DES or PROGRESS page for the predicted distance before assuming the FMC has failed.
How do I restore a missing PMDG descent path?
Restore the route and performance calculation before changing autopilot modes.
- Confirm that the route is active. The CDU should show an active route rather than an unexecuted
MODroute. Check that the correct origin and destination are present, then press EXEC for legitimate pending changes. - Complete PERF INIT. Enter or verify zero-fuel weight or gross weight, reserves, cost index and cruise altitude. A wrong or blank cruise altitude is one of the most common reasons VNAV cannot construct a sensible profile. Our 737 FMC entry sequence explains which route and performance entries drive VNAV.
- Walk through every LEGS page. Follow the route from the active leg to the destination. Remove unintended discontinuities, duplicate fixes and routing loops, but do not join a deliberate vectors-to-final gap unless that is the route you have been cleared to fly.
- Verify the arrival and runway. A runway is not always required merely to estimate a descent, but selecting the intended STAR, approach and runway gives the FMC a meaningful endpoint and the correct altitude restrictions. Execute the selection and recheck the resulting legs.
- Inspect every altitude constraint. Look for a restriction above cruise altitude, below a plausible approach altitude or entered with the wrong format. Do not delete valid published constraints simply to force a path; correct only entries that are clearly wrong.
- Check imported routes carefully. Importing a route does not necessarily initialise performance data, select an arrival or execute every modification. Use our checks for a complete PMDG 737 SimBrief import if the problem began with an imported plan.
- Allow the FMC to recalculate. After correcting and executing the route, check the DES and LEGS pages again and increase the ND range. If the new
T/Dis behind the aircraft because the route was changed late, the original descent point cannot be recovered.
Do not erase the entire route or reinitialise the FMC in flight as a first response. Correcting the specific broken leg, arrival or performance entry is safer and preserves the remaining lateral route.
Why is T/D displayed but the PMDG aircraft will not descend?
If T/D is displayed, the FMC has calculated a descent path; the usual problem is MCP clearance or vertical-mode selection.
Before reaching top of descent, set the MCP altitude to the next cleared lower altitude. The MCP acts as an altitude limit, so VNAV will not descend through an altitude still set in the window. Merely turning the altitude selector does not command a descent unless the correct vertical mode is armed or active.
Read the Flight Mode Annunciator rather than relying on illuminated buttons. VNAV PTH should follow the computed profile, while altitude hold indicates that the aircraft is still maintaining level flight. If VNAV does not arm or engage, work through our focused LNAV and VNAV fault sequence.
If the MCP was lowered after passing T/D, the aircraft may require altitude intervention, a DES NOW prompt where provided, or another descent mode. The available command and its exact behaviour depend on the PMDG aircraft.
Why does the FMC say DES PATH UNACHIEVABLE?
DES PATH UNACHIEVABLE means the path exists, but the aeroplane cannot meet it with its present altitude, speed, wind or configuration.
Common causes include a late descent clearance, a strong tailwind, an unexpectedly low restriction, excessive speed or a route shortcut that removed descent distance. Recheck the constraint first. If it is valid, use permitted drag and an appropriate descent mode rather than forcing an extreme vertical speed.
- VNAV PTH: choose it when the route and constraints are valid and the aircraft can capture the computed profile.
- LVL CHG: use it when a positive, speed-controlled descent towards the MCP altitude is needed.
- V/S: use it for a measured correction or path interception, while monitoring airspeed closely.
Can I continue without an FMC descent path?
Yes, but the descent must be planned and monitored manually until a valid VNAV path is available.
Use distance-to-go, altitude restrictions and a suitable vertical mode; do not chase the missing path with abrupt pitch commands. Our 737 descent and approach setup covers path recovery, MCP altitude management and choosing between VNAV, LVL CHG and V/S.