B737 Limitations

The limitations of the Boeing 737 are the boundaries within which the airplane is certified and must be operated: weights, altitudes, winds and runway slope, the use of the autopilot and autoland, engine and APU operation, and the behavior of individual systems. They come from the airplane flight manual (AFM), and the flight crew operations manual (FCOM) gathers them, with related operating information, in its Limitations chapter. Several are memory items, applied without opening a book.

This article follows a 737 NG FCOM whose figures vary between airplanes of the fleet it covers; where they do, the value for most airplanes is given with the alternatives. Where the EASA type certificate data sheet (TCDS) or the FAA Flight Standardization Board (FSB) report confirms a 737 MAX figure, it is noted. The operator's current AFM and FCOM always govern. How the documents fit together is described in B737 flight deck and operating documentation.

Limitations and how the FCOM presents them

The Limitations chapter combines three kinds of content: AFM limitations, AFM operational information and non-AFM operational information, the last being Boeing guidance that is not part of the certified flight manual. An item is included if it is operationally significant, is required by an FAA airworthiness directive or is required by another regulatory requirement. It is left out if it is already built into an FCOM procedure or shown on a placard, display or other marking. That is why the chapter's entry for maximum speeds says only to observe the gear and flap placards, and why engine limits are read from the displays, where maximum and minimum limits are red and caution limits amber.

Items marked # are memory items: the crew must comply with them without referring to the manual. If the AFM and the FCOM disagree on a weight limitation, for example because their revisions were released on different dates, the AFM governs. Some values apply only to operation under FAA rules and some only to particular airplanes; an operator under another authority, or with a different fit, uses the figures in its own AFM.

Weights and operating envelope

The weight limits depend on the weight option the airplane was delivered with:

The heaviest takeoff option matches the 79,015 kg (174,200 lb) that the EASA TCDS gives as the maximum takeoff weight of the 737-800. The same document gives 82,645 kg (182,200 lb) for the 737-8 and 88,314 kg (194,700 lb) for the 737-9; these are certified maxima, and an airplane's AFM may show a lower option. The limit that binds on a given day is often a performance limit rather than a structural one (see maximum structural and regulated masses).

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The maximum operating altitude is 41,000 ft pressure altitude, the same for the NG and the MAX, and the maximum takeoff and landing altitude is 8,400 ft pressure altitude. Flap extension is not attempted above 20,000 ft, where the higher Mach number would put excessive structural loads on the flaps. The severe turbulent air penetration speed, a memory item for the climb and descent, is 280 KIAS or M 0.76, whichever is lower; in the cruise the Severe Turbulence supplementary procedure applies.

For maximum speeds the PFD does the work: the bottom of the red and black maximum operating speed bar on the speed tape marks the lowest of VMO/MMO, the landing gear placard speed and the flap placard speed. The FLAPS LIMIT placard in the NG FCOM gives 250 kt for flaps 1, 2 and 5, 210 kt for 10, 200 kt for 15, 190 kt for 25, 175 kt for 30 and 162 kt for 40. The FSB lists revised flap placard speeds for the 737-9, so the placard in the airplane flown governs.

The maximum flight operating latitude follows from the magnetic variation tables in the ADIRUs. On most airplanes it is 82°N and 82°S, reduced to 70°N between 80°W and 130°W and to 60°S between 120°E and 160°E. Airplanes with a different table are restricted instead to 73°N between 80°W and 170°W (see polar and grid navigation).

Runway slope, tailwind and crosswind

The maximum runway slope for takeoff and landing is ±2 %, uphill or downhill. The maximum tailwind component for takeoff and landing is 10 kt, a memory item. For the 737-8 the EASA TCDS records 15 kt, but as a capability demonstrated for takeoff and for manual and automatic landings, not as an operational approval above 10 kt.

The only crosswind figures among these limitations are the autoland limits below. For manual takeoffs and landings the crosswind the crew may accept is the operator's limit; a maximum demonstrated crosswind is not itself a limitation unless the AFM lists it as one (see wind components and crosswind limits). Runway condition enters elsewhere: assumed temperature reduced takeoff thrust is not permitted with the antiskid inoperative, and is not used on runways contaminated with standing water, slush, snow or ice (see reduced and derated takeoff thrust).

Autopilot, autoland and single-channel limits

The autopilot flight director system limits, most of them memory items:

• The autopilot is not engaged below 400 ft AGL after takeoff.
• Aileron trim is not used with the autopilot engaged.
• In single channel operation, with one autopilot engaged, the autopilot must not remain engaged below 50 ft AGL on an approach (a figure for operation under FAA rules). A single autopilot has no automatic flare and touchdown capability and no autopilot go-around; both need two autopilots engaged in APP mode, the second in CMD by 800 ft RA.
• LVL CHG is not used on final approach below 1,000 ft above field elevation, a non-AFM item.

For autoland, the FCOM gives these limits; the wind limits, and one-engine autoland where it is allowed, apply to operation under FAA rules when the landing weather minima are predicated on autoland:

Some airplanes have a fail-operational autopilot. On them, autopilot and flight director ILS approaches, and GLS approaches where installed, that use magnetic-north-referenced courses are prohibited where the IRS magnetic variation table errors exceed 3°, against 5° for magnetic heading operations in general. At the MAX's return to service, EASA based its AFM's autopilot minimum use height on a one-second recognition time instead of the 0.5 s used at the original validation, and prohibited RNP AR operations (see autoland and B737 autopilot flight director system).

Engine and APU limits

Engine ignition must be on for takeoff, landing, operation in heavy rain and anti-ice operation. On the engine displays the red EGT line is the maximum takeoff EGT, or the maximum in-flight start EGT when the start limit redline is not shown, and the lower end of the amber band is the maximum continuous EGT. The engine start procedure adds a starter duty cycle: each start attempt is limited to 2 minutes of starter use, with at least 10 seconds between attempts; extended motoring is limited to 15 minutes for the first two attempts, with 2 minutes between them, and to 5 minutes for the third and later attempts, with 10 minutes between them.

The APU altitude limits are memory items:

The APU bleed valve must be closed whenever air from another source could flow back through it: with ground air connected and the isolation valve open, with the engine No. 1 bleed valve open, or with the isolation and engine No. 2 bleed valves both open. It may be open during an engine start, but engine power above idle is then avoided (see B737 APU).

System limitations

Air systems and flight controls. With one or both engine BLEED switches ON, the packs are not operated in HIGH for takeoff, approach or landing; the CARGO FIRE and SMOKE/FUMES REMOVAL non-normal checklists override this, because they select HIGH for ventilation. Two pressure relief valves limit the cabin differential pressure to 9.1 psi. Holding in icing conditions with any flaps extended is prohibited, and rapid, large alternating control inputs are avoided even below maneuvering speed: both are memory items. In flight the SPEED BRAKE lever is not moved beyond the FLIGHT DETENT, and the speedbrakes are not deployed below 1,000 ft radio altitude. The electric alternate flaps need 15 seconds after the ALTERNATE FLAPS switch is released before it is moved again, and 5 minutes of cooling after a full cycle from 0 to 15 and back.

Fuel. The maximum tank fuel temperature is 49°C. The minimum, before takeoff and in flight, is −43°C or 3°C above the fuel's freezing point, whichever is higher, and a fuel system icing inhibitor does not change it. Any scheduled imbalance between main tanks 1 and 2 must be zero, and a random imbalance must not exceed 453 kg; the main tanks must be full if the center tank holds more than 453 kg; and a center tank pump is never run dry intentionally, with its LOW PRESSURE light on. The EASA TCDS gives the MAX the same temperature limits and prohibits wide-cut fuel such as Jet B or JP-4 (see B737 fuel system).

Communications. HF radios are not operated during refueling, a memory item, and flights predicated on the HF frequencies 29.489 and 29.490 MHz are prohibited. ACARS is limited to messages that would not create an unsafe condition if improperly received; messages such as pre-departure and oceanic clearances, digital ATIS and takeoff data may be used if verified by approved procedures. Protected mode CPDLC is for the cruise phase and non-critical messages only (see controller-pilot data link communications). On the ground, primary ATC communication uses the VHF radio connected to the top fuselage antenna.

Navigation and surveillance. The ADIRUs are not aligned above 78°15' of latitude, where the horizontal component of the earth's rotation, which the ADIRU senses to find true north, is too small for an accurate alignment. Operations based on magnetic heading or track are prohibited where the IRS magnetic variation table errors exceed 5°. The terrain display is not used for navigation, and the look-ahead terrain alerting and display are not used within 15 NM of takeoff, approach or landing at an airport or runway not in the GPWS database. The weather radar is not operated in a hangar, except in test mode (a memory item), nor while people are in the hazard area in front of the nose radome. Where fitted, RAAS callouts are not used for navigation or in place of NOTAM and ATIS information.

Doors and landing gear. Before a departure with passengers, the handle covers on the overwing exits must be verified installed; the flight deck door access system is checked once each flight day; on revenue flights the escape slide girt bars are installed for taxi, takeoff and landing. The brakes are not applied until after touchdown.

RVSM altimetry tolerances

Reduced vertical separation minima (RVSM) allow 1,000 ft of vertical separation between FL290 and FL410 inclusive, instead of 2,000 ft. Approved airplanes carry two independent altimetry systems, an automatic altitude hold and an altitude alert whose threshold does not exceed ±300 ft. The 737 limitations add checks on the altitude displays themselves:

The standby altimeters do not meet the accuracy requirements of RVSM airspace, so RVSM operation relies on the two primary displays. The ground checks compare them with each other and with the known field elevation before departure, so that an error is found before the airplane climbs into airspace where 1,000 ft is the whole of the separation (see RVSM and pressure altimeter).

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.