B737 VNAV and LNAV

LNAV (lateral navigation) and VNAV (vertical navigation) are the modes in which the Boeing 737's autopilot flight director system (AFDS) and autothrottle follow the flight management computer (FMC) instead of the targets set on the mode control panel (MCP). LNAV steers along the active route. VNAV flies the FMC's climb, cruise and descent profile, and the way it shares the work between pitch and thrust changes with the phase of flight. Together they fly most of a normal line flight: Boeing's normal procedures assume full use of the automated features, LNAV, VNAV, autoland, autopilot and autothrottle among them.

This article follows a 737 NG flight crew operations manual (FCOM), with 737 MAX differences where FAA or EASA documents confirm them. It builds on B737 autopilot flight director system, B737 FMC and CDU and B737 autothrottle and thrust management, applying their rules to worked cases phase by phase. The operator's FCOM governs.

LNAV engagement and capture

On the ground, LNAV arms only when the origin runway is in the flight plan, an active route is entered, the track of the first leg is within 5° of runway heading and both flight director switches are ON, and only if LNAV is selected before TO/GA. Guidance becomes active at 50 ft AGL. With a runway heading of 274° and a first leg tracking 268°, LNAV arms; with a first leg of 282°, 8° away, it does not, and LNAV is selected in the air above 400 ft RA, where the takeoff procedure calls for a roll mode anyway, once the in-flight criteria below are met.

In flight, LNAV engages at once if the airplane is within 3 NM of the active route leg, whatever its heading. Further out it engages only on an intercept heading of 90° or less that meets the leg before the active waypoint. An airplane 6 NM from the route on a 100° intercept will not capture; turned onto a 45° intercept that meets the leg short of the active waypoint, it will.

LNAV disconnects at the end of the active route, at a route discontinuity, when the airplane intercepts a selected approach course with VOR/LOC armed, when HDG SEL is selected, and when the capture criteria are lost.

LNAV follows the FMC position, so an error in position becomes an error in track. Actual navigation performance (ANP) is the radius within which the FMC is 95 % certain the airplane lies; when it exceeds the displayed required navigation performance (RNP), UNABLE REQD NAV PERF-RNP appears after the time to alert, with the amber FMC lights. An undetected across-track map shift leaves the airplane on a track offset from the one intended; an along-track shift moves the altitude changes earlier or later.

VNAV climb

VNAV arms on the ground with a valid flight plan, performance data entered and executed and both flight director switches ON, and becomes active at 400 ft AGL; it is inhibited below 400 ft RA or when the performance initialization is incomplete. The crew arm it on the MCP in the Before Start flow and verify above 400 ft RA after takeoff that it has engaged.

In the climb the autothrottle holds the FMC thrust limit in N1, and the AFDS holds the FMC target speed with pitch: the LVL CHG pairing, flown to the FMC speed schedule and constraints, with the IAS/MACH window blank. Engaging VNAV in the climb engages N1 automatically, whereas LVL CHG does not engage N1 automatically during the first 2½ minutes after lift-off. If the climb speed profile cannot meet an altitude restriction, the CDU scratchpad shows UNABLE NEXT ALTITUDE.

The MCP altitude remains the clearance: VNAV levels the airplane at the MCP altitude or at the top of climb, whichever comes first. A worked case: a departure carries a constraint of FL80 at a waypoint, and ATC clears the airplane to FL230 with no restriction. The crew set FL230 on the MCP and push ALT INTV, which deletes the lowest FMC altitude constraint below the MCP altitude, the FL80 one, and VNAV continues the climb.

VNAV cruise

In cruise the autothrottle holds the FMC target speed in FMC SPD and the AFDS holds the FMC altitude. The target is ECON speed, set by the cost index, unless the crew choose long range cruise or enter a speed; a cost index of zero gives maximum range cruise (see cost index and cruise economics).

ALT INTV works in cruise too. With the MCP altitude set above the FMC cruise altitude, a push resets the cruise altitude to it and starts a cruise climb. More than 50 NM from the top of descent, with a lower MCP altitude, a push starts a cruise descent; if the MCP altitude is set below a descent constraint, the FMC changes instead to EARLY DES and honors that constraint.

VNAV's predictions depend on gross weight: if the fuel quantity becomes invalid, VERIFY GW AND FUEL appears, the FMC keeps the last valid figure for its predictions and VNAV, and the crew enter and keep updating an estimated fuel weight.

VNAV descent: PTH and SPD

The FMC calculates the top of descent (T/D) back from an end of descent (E/D) point, and shows it on the map as a green circle labeled T/D. Without an E/D its predictions assume a descent to 1,000 ft above the destination field elevation, and a VNAV path descent is not available. The path assumes deceleration to reach the final approach fix or glideslope intercept at VREF 40 plus 20 kt.

VNAV descends in one of two modes:

Engaging VNAV for the descent puts the autothrottle in RETARD, then ARM once the levers reach idle. A vertical angle leg that becomes active during a speed descent changes VNAV SPD to VNAV PTH automatically. Passing the T/D with a target speed within about 6 kt of VMO/MMO, VNAV may revert to LVL CHG to prevent an overspeed; the crew reduce to the VNAV target descent speed before engaging VNAV again.

The normal descent target is ECON speed above the speed restriction altitude and 240 kt below it, until the deceleration for the approach. With a 250 kt restriction at 10,000 ft, an airplane descending at 290 kt through 11,000 ft is held at the restriction altitude until its speed is at or below 260 kt, the restricted value plus 10 kt; it then continues at its 240 kt target.

An early descent, before the T/D, is flown at 1,000 fpm until the idle path is intercepted if a path descent is planned, or at idle thrust if a speed descent is planned. LNAV may be disengaged during the descent with VNAV remaining in path whatever the cross-track error, with one exception: on an active leg with a database vertical angle, with LNAV not engaged, VNAV ends if the cross-track deviation exceeds twice the RNP.

The FMC's T/D can be crosschecked with the rule of about 3 NM for every 1,000 ft to lose (see climb and descent planning). From FL370 to an airport near sea level, 37 × 3 = 111 NM, plus distance to slow down. Late, with 12,000 ft to lose in 30 NM, the airplane needs 400 ft per NM against roughly 333 ft per NM on an idle path: it is high, and speedbrakes or extra track miles are needed (see descent performance). The speedbrake lever is not moved beyond the FLIGHT DETENT in flight, and the speedbrakes are not deployed below 1,000 ft radio altitude (see B737 limitations).

Speed and altitude intervention

With VNAV engaged, pushing SPD INTV makes the IAS/MACH window alternately show the selected speed and blank: while it shows, speed intervention is active; while it is blank, the FMC target speed is. ALT INTV changes the altitude constraints instead. Their effects depend on the phase:

A worked case: descending on an arrival with a constraint of FL150 at a waypoint, the airplane is cleared to FL100 with the restriction canceled. With FL100 set on the MCP, one push of ALT INTV deletes the FL150 constraint, the highest above the MCP altitude. When ATC instead assigns a speed on a normal path descent, SPD INTV leaves VNAV in VNAV PTH, whereas selecting LVL CHG would give up the path.

VNAV on approach and IAN

The FMC goes on approach within 2 NM of the first approach waypoint, approach transitions such as arcs and procedure turns included, or within 2,000 ft of airport elevation, whichever comes first. Two features then apply: with the MCP altitude set at least 300 ft above the airplane for the missed approach, VNAV stays in VNAV PTH; and if the airplane is more than 200 ft below the vertical path, VNAV commands zero vertical speed until it intercepts the path.

In the FCOM landing procedure with VNAV, about 2 NM before the final approach fix, with ALT HOLD or VNAV PTH annunciated, the crew set the DA(H) or MDA(H) on the MCP, select or verify VNAV and use speed intervention as needed. The missed approach altitude goes on the MCP once the airplane is at least 300 ft below it: with a missed approach altitude of 3,000 ft, at or below 2,700 ft. With suitable visual reference at minimums, the autopilot is disengaged as the regulations require and the autothrottle with it, and the glide path is maintained to landing.

A barometric VNAV path is only as good as the altimeter setting and the database. The FMC's vertical ANP (VANP) is its 99.7 % estimate of altitude error, valid only with the setting from the ATIS or the approach clearance, and its default vertical RNP is 400 ft in the oceanic, en route and terminal phases. LNAV/VNAV and IAN are not authorized with QFE. The procedure is retrieved by name from the navigation database, not entered as a series of waypoints (FAA AIM), and on IAN approaches no waypoints are added to it by hand. Cold air puts an uncompensated baro-VNAV airplane below the designed path (see RNP approaches).

On airplanes with integrated approach navigation (IAN), FMC approaches give glide path (G/P) and final approach course (FAC) deviations and alerts displayed like an ILS, so a non-ILS approach is flown with ILS-like procedures. IAN is used only for approaches with a published glide path angle on the LEGS page for the final segment, a runway waypoint (RWxx) or a missed approach point before the runway, and is not recommended where a visual maneuvering segment of the approach is not in the FMC database. About 2 NM before the final approach fix the crew call "APPROACHING GLIDE PATH", with gear down and flaps 15; at the fix they verify the crossing altitude and that the altimeters agree within 100 ft. If the autopilot is still engaged below 100 ft RA in FAC or G/P, "AUTOPILOT, AUTOPILOT" sounds and an amber AUTOPILOT flashes on the attitude display.

VNAV ends on glideslope capture, when another pitch mode is selected, at the end of the LNAV route, or when the glideslope intercept waypoint is passed with G/S armed.

Common traps

• Speed protection. Autothrottle approach speed protection works only in non-VNAV modes. There is no minimum speed reversion from VNAV with the flaps beyond 12.5, nor on a VNAV PTH level segment, while the autothrottle wake-up, at 5 kt below the command speed, exists only in VNAV.
• The 737 MAX software change. According to the FAA review, in the modes without active speed protection, among them VNAV PATH (ALT HLD) and, with flaps 15 or more, VNAV PATH (V/S), the updated MAX autopilot disconnects 1 second after stick shaker and the flight director bars bias out of view. When guidance returns, the pitch mode is LVL CHG after ALT HLD or VNAV PATH (ALT HLD), and unchanged after G/S, G/P, V/S or VNAV PATH (V/S).
• FMC changes. Moving the FMC source select switch, an FMC power interruption or a failure of the left FMC disengages LNAV and VNAV. After a software exception (FMC U10.8 and later), the downgrade mode keeps LNAV and the map but disconnects VNAV, and VNAV INVALID - PERF asks for the cost index to be entered and executed again.
• RNP AR on the MAX. EASA's return to service of the 737 MAX added an AFM limitation prohibiting RNP AR operations.

The flight mode annunciations are the final word: a mode change is boxed for 10 seconds after it engages, and the crew check each change there rather than trusting the switch they pressed (see flight deck automation).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.