VNAV Descent Troubleshooting: Why Isn't It Descending?

The same short Chicago–Cleveland leg (~275 nm) you set the descent up on. Everything in this drill is a CDU diagnosis, so the route has to exist before the descent pages have anything to reason about.

J60 to DJB — the Dryer VOR/DME west of Cleveland — gives the descent a real fix to be planned around. On a 737 airways are entered in the VIA/TO columns of the RTE page, never on LEGS, and an airway must contain the TO waypoint of the line before it (FCOM §11.40.17) — hence the DIRECT to JOT (Joliet) first.

FL290 is a normal eastbound level for this sector. CRZ ALT is the top of the FMC vertical profile: the descent path is computed from here down to the end-of-descent altitude, so the FMC needs both numbers before it can build a path at all.

The A suffix means at or above, B means at or below: "An altitude followed by A or B signifies a requirement to be at or above or at or below that altitude at the waypoint" (FCOM SP.11.17). This is the constraint that will trap the descent: VNAV honours it, so the aeroplane levels off at FL240 and stays there until DJB is behind it — "automatic level-off occurs at MCP altitude or VNAV altitude, whichever is reached first" (FCOM 4.10.6). At-or-above is the one crews misread, which is why the FCOM warns that "an At or Above is not an altitude restriction but an At or Below is a restriction" and that confusion here has the potential to lead to a level bust.

E/D ALT reads FL240 — the lowest altitude anywhere in the FMC descent profile. Two floors stop a 737 descending. The first is the MCP altitude window: VNAV will not descend through it, and "selecting a lower MCP altitude arms FMC to automatically begin descent upon arrival at FMC top of descent point" (FCOM 4.10.6) — so a window still sitting at cruise means no descent at all. That is a panel knob, not a CDU field, and it is the first thing you check in the cockpit. The second floor is an FMC constraint, and that half the CDU will tell you: this profile bottoms out at FL240, nowhere near your clearance.

A new restriction at a waypoint "changes any prior speed and altitude restriction at this waypoint" (FCOM SP.11.17): the FL240A is gone, replaced by at-or-below 10,000. On the flight deck there is a second route to the same result — dial the MCP down and press ALT INTV, which deletes one FMC altitude constraint per push (FCOM SP.11.16). The trainer has no MCP, so you fix it where the FMC actually stores it: on the LEGS page.

E/D ALT now reads 10000. It moved because the LEGS constraint moved — the DES page displays the profile, the LEGS page owns it. That before/after pair is the whole diagnostic: if E/D ALT sits above the altitude you have been cleared to, the FMC is still holding a constraint you have not dealt with.

DES NOW arms an early descent: DES NOW ARMED appears in amber, the title flips to MOD ECON PATH DES and the EXEC light comes on. Nothing has happened to the aeroplane yet — arming is not executing, and this is where a rushed crew stops and wonders why they are still level.

On a path descent, executing DES NOW "initiates a VNAV descent of 1000 feet per minute at ECON speed. Upon reaching the planned descent path, VNAV transitions to maintain the planned descent path" (FCOM 11.42.27). Read the pitch annunciation from there: VNAV PTH means the AFDS is tracking the FMC path with pitch and the thrust is at idle; VNAV SPD means pitch is holding the target speed instead. High on the path in VNAV PTH the aeroplane holds the path with pitch and the speed runs up instead: at 10 knots above the FMC target speed — or within 5 knots of Vmo/Mmo — the CDU posts DRAG REQUIRED, and the answer is speedbrake, not more pitch (VNAV disengages if a limit speed would be exceeded). And if every FMC altitude constraint gets deleted, "the descent mode will revert to a VNAV speed descent" (FCOM SP.11.16).