Intercept From Vectors: Rebuilding a Plan That Will Sequence

Denver to Seattle, about 890 nautical miles direct — a routine 737 domestic leg that arrives at Sea-Tac from the south-east over the Cascades. Everything in this drill happens in the last hundred miles, but the FMC needs an origin and a destination before any arrival page has something to reason about.

CHINS is a real Sea-Tac arrival from the south-east, and it is the reason this drill exists. Its published route runs CHINS to RADDY to HUMPP to AUBRN — and then it stops flying to fixes altogether: "LANDING NORTH: HEADING 250 FOR VECTORS TO FINAL APPROACH COURSE" (FAA STAR form 8260-17.1). That last leg is a heading, not a track to a place. The FMC shows heading legs as (xxx° HDG) above the waypoint line (FCOM 11.41.13), and what such a leg flies toward is a condition rather than a location — "a conditional waypoint is not necessarily associated with a land reference" (FCOM 11.31.3). One honest limitation before you go on: this trainer records your arrival selection but does not expand the STAR fixes onto the LEGS page, so RADDY goes onto the route by hand in a moment.

This box-prompt gap is what a vectors leg leaves behind. A heading leg has no fix at the end of it, so the FMC cannot link it to whatever comes next and marks the join as open — "box prompts are displayed for route discontinuities" (FCOM 11.41.13). LNAV will not fly across a gap: it navigates to the last good waypoint and then has nothing to sequence to, which is precisely the state you are in when the vectors start and the map goes quiet. The FMC is not broken and the arrival is not mis-coded — the procedure genuinely ends with "and now ATC will tell you". What it costs you is real, though: with no route ahead there are no predictions ahead either, which is why the Ryanair FCOM advises that selecting "direct to an appropriate waypoint when on radar vectors provides more accurate descent information". Everything from here is about giving the FMC a route to sequence before you need it.

J20 is the jet route in from the east — Pendleton, then Yakima, then RADDY, then Seattle — and RADDY is on the CHINS arrival as well: "FROM CHINS ON YKM R-284 AND SEA R-104 TO RADDY" (FAA STAR form 8260-17.1). A fix that sits on both your enroute structure and the arrival is exactly the fix a controller hands you back to. On a 737 airways are entered in the VIA and TO columns of the RTE page, never on LEGS (FCOM 11.40.17-18), and an airway must contain the TO waypoint of the line before it — so the route goes DIRECT to FQF, the Falcon VOR at J20's Colorado end, and joins there. You rebuilt the route from scratch, so it is not active until you activate it: ACTIVATE and EXEC make it the active route (FCOM 11.40.18), and every change to it from here is a modification you execute. Row 2 of the LEGS page now reads RADDY where the box prompt was.

Straight off the chart: "TURBOJET: LANDING NORTH: CROSS RADDY AT 12000 AND AT 250 KIAS. MAINTAIN 12000" (FAA STAR form 8260-17.1). No A or B suffix — this one is a mandatory AT, not an at-or-above or an at-or-below — and the slash is not optional: a speed and an altitude typed without it fails every entry format and returns INVALID ENTRY (FCOM 11.31.36-37). It matters more here than it looks. Vectors do not delete a crossing restriction; they only take away the route that was going to fly it for you, and the restriction is still yours to make. Entered restrictions display in large font and the FMC's own computed values in small (FCOM 11.41.13) — on the aeroplane that one difference tells you at a glance which numbers a human committed to and which the box guessed. This drill skips PERF INIT, so the other rows here show dashes rather than predictions.

"Existing waypoints can be copied from a RTE LEGS page into the scratchpad by pushing the line select key adjacent to the desired waypoint" (FCOM 11.42.1-2). Building the habit of line-selecting rather than typing is worth real money on a busy arrival: a fix that came off your own route cannot be fat-fingered, cannot turn out to be a duplicate ident three states away, and takes one press instead of five. It is the same mechanism that closes a gap by hand — "any waypoint from the route can be copied into the scratchpad to remove the discontinuity" (FCOM 11.42.7).

"To fly direct to a waypoint or intercept a course to a waypoint, enter the waypoint name on RTE LEGS page 1 active waypoint line" (FCOM 11.42.8). The title reads MOD RTE 1 LEGS, the white EXEC light is on, and the staged fix sits on the DIRECT TO line, drawn white: an unexecuted modification is shaded white on this unit. Nothing has reached the autopilot: this is the moment to read the clearance back and let the other pilot look at it. It is also where the real aeroplane offers a second half this scenario does not drill. On the aircraft the INTC CRS prompt appears at line 6R as soon as the active waypoint is modified, and a course entered there — "valid input is any course from 000 through 360" (FCOM 11.42.8-9) — makes the FMC draw the extended inbound course to the fix instead of a straight line from wherever you happen to be, so you can be vectored onto that course and let LNAV capture it. That is the technique the FCOM calls "Intercepting a Leg (Course) to a Waypoint" (FCOM SP.11.5), and on vectors it is the recommended one: "intercept course modifications may be used to join the LNAV path at any point on the initial, intermediate or missed approach segments" (FCOM SP.4.9). The real unit gives it a dedicated key, DIR INTC, and this trainer has it too: DIR INTC opens LEGS with INTC LEG TO boxes at 6R, which take the fix and then the INTC CRS course. The line-select path this scenario drills gets the same field: look at 6R now and it reads INTC CRS, exactly as on the aircraft. This step leaves it empty and flies the plain direct; typing a course there first would make it an intercept.

"Observe MOD RTE LEGS page changes to ACT" (FCOM SP.11.5). That title, plus an extinguished EXEC light and an empty DIRECT TO line, IS the verification: nothing staged, nothing pending, and a route that runs from present position to RADDY with its 250/12000 restriction and on to the destination — a plan that will sequence. Two things to carry to the aeroplane. First, direct is not always legal: "Direct To modifications are not permitted when the fix is the beginning of an RF leg" or "the fix is the Final Approach Fix (FAF) for the procedure" (FCOM SP.4.9), which is why a clearance sends you to an arrival fix like RADDY rather than straight to the FAF. Second, executing an intercept can drop the very mode you were rebuilding for: "LNAV may disengage after execution of an intercept leg to a waypoint. If LNAV disengages, turn to a heading to satisfy LNAV capture criteria... and then engage LNAV" (FCOM SP.11.5). Those criteria are worth knowing cold — "on any heading and within 3 NM of the active route segment", or, outside 3 NM, "on an intercept course of 90 degrees or less" that meets the segment before the active waypoint (FCOM 4.20.7). Until one of them is satisfied, HDG SEL is still doing the flying — which is exactly where this started.