Learn when the Boeing 737 autothrottle controls thrust or speed, how MCP and VNAV modes differ, and how to fix common simulator errors.
In a Boeing 737 simulator, arm A/T, select the required MCP or VNAV speed, then engage the vertical mode that suits the manoeuvre. Read the flight-mode annunciator: in MCP SPD or FMC SPD the autothrottle controls speed; in N1 or idle-thrust modes, the elevators usually control speed.
The IAS/MACH number in the mode control panel is only a target. It does not make the aircraft chase that speed unless the active pitch and autothrottle modes use it. The flight-mode annunciator, or FMA, across the top of the primary flight display is the final authority.
What controls speed in each Boeing 737 mode?
Speed can be controlled by thrust, pitch, or a combination of pitch and drag, depending on the active mode.
| FMA or vertical mode | How speed is controlled | What to expect |
|---|---|---|
| MCP SPD | Autothrottle varies thrust | Maintains the speed selected in the MCP, commonly with ALT HOLD, V/S or approach modes. |
| FMC SPD | Autothrottle varies thrust | Maintains a speed calculated or constrained by the FMC. |
| VNAV SPD | Pitch controls speed | Autothrottle normally commands a thrust limit such as N1 or idle while pitch follows the FMC speed. |
| LVL CHG | Pitch controls MCP speed | Autothrottle applies climb thrust or retards towards idle; pitch holds the selected speed. |
| VNAV PTH | Pitch follows the vertical path | Thrust manages speed when available, but an idle descent may require speedbrake or earlier configuration. |
| V/S | Autothrottle attempts to hold speed | Pitch holds the selected vertical rate. Speed will drift if the demanded rate exceeds available thrust or drag. |
| THR HLD | Autothrottle servos release the levers | Normally seen during take-off. It is an expected mode, not an autothrottle failure. |
| ARM or RETARD | No normal active speed hold | ARM means the system is standing by; RETARD drives the levers towards idle. |
How do I operate the autothrottle correctly?
- Arm the system. Move the A/T ARM switch to ARM. This only makes the autothrottle available; it does not by itself command thrust or hold speed.
- Set the speed source. Select an IAS or Mach value in the MCP, or programme valid performance and speed data in the FMC for VNAV. Our 737 FMC programming explanation covers how those calculated targets reach VNAV.
- Choose the appropriate mode. Use TO/GA for take-off, VNAV for an FMC-managed profile, LVL CHG for a speed-controlled climb or descent, or SPEED with a compatible pitch mode when thrust should maintain the MCP speed.
- Confirm the FMA. After every button press, check the active green mode and the armed white mode. A lit MCP button shows what you selected; the FMA shows what the aircraft actually accepted.
- Monitor the result. Check airspeed trend, thrust, pitch and configuration. Autothrottle cannot overcome an impossible V/S demand, insufficient drag, maximum thrust or a badly configured approach.
During take-off, arm A/T and activate TO/GA using the control provided by the aircraft add-on. A typical simulation then shows N1 followed by THR HLD. Do not press SPEED simply because you want take-off speed maintained; the take-off flight-director mode controls pitch while autothrottle manages take-off thrust.
What does the SPEED button actually do?
The SPEED push-button commands the autothrottle to maintain the MCP-selected IAS or Mach when the active flight mode permits it.
- Turning the IAS/MACH selector changes the target shown in the speed window.
- Pressing SPEED normally engages MCP SPD; it does not engage the autopilot or select a climb or descent mode.
- Pressing the speed-selector knob may activate speed intervention while VNAV remains engaged, if that function is modelled and available in the simulated 737.
- The C/O control changes the display between indicated airspeed and Mach. It is not a speed-hold button.
Speed intervention opens the MCP speed window so that an ATC-assigned speed can replace the FMC target without abandoning the entire VNAV profile. Implementations differ between 737 variants and simulator add-ons, so confirm the resulting FMA rather than assuming the button press worked.
Should I use VNAV, LVL CHG or V/S?
Choose the vertical mode according to whether speed, vertical path or rate of climb is the priority.
- VNAV: use it for a programmed climb or descent containing FMC speeds and altitude constraints. VNAV SPD controls speed with pitch; VNAV PTH prioritises the calculated vertical path.
- LVL CHG: use it when you want a direct climb or descent while holding the MCP speed. It normally commands climb thrust going up and near-idle thrust going down.
- V/S: use it for a specific vertical rate or a small path correction. Watch speed closely because the autothrottle may reach idle or maximum thrust without maintaining the selected target.
- ALT HOLD with SPEED: use this combination in level flight when the MCP speed should be maintained through thrust changes.
A descent that is high, fast or constrained needs more than a different button. Reduce the selected speed early, use appropriate drag and avoid forcing a steep V/S that the autothrottle cannot support. We cover those decisions in our 737 descent and approach setup procedure.
Why is the Boeing 737 ignoring my selected speed?
The usual cause is a mode mismatch: the selected number is visible, but the active mode is following an FMC target, vertical path or thrust limit instead.
- The FMA says ARM: A/T is armed but has no active thrust command. Engage a compatible mode and check the FMA again.
- The speed window is blank: VNAV is probably using an FMC speed. Use speed intervention if available, or select another suitable vertical mode.
- The FMA says N1: the autothrottle is controlling thrust, not airspeed. VNAV SPD or LVL CHG must use pitch to maintain speed.
- Speed drifts in V/S: the selected vertical rate requires more thrust or drag than the aircraft has. Reduce the rate instead of repeatedly changing the speed target.
- The thrust levers hunt or jump: remove duplicate throttle assignments, calibrate the hardware axis and add a small dead zone if the physical quadrant is sending noise. Avoid moving a fixed hardware throttle while A/T is active unless the add-on provides suitable override logic.
- A mode will not engage: required FMC data, route conditions or interception criteria may be missing. Our LNAV and VNAV fault guide explains the associated FMA indications.
Altitude capture also changes the control logic. For example, LVL CHG may transition through altitude acquisition into altitude hold, with the autothrottle changing from N1 or idle behaviour to MCP SPD. This is normal, but it can look like an uncommanded thrust change if the FMA is ignored.
Should autothrottle remain engaged for landing?
Autothrottle can remain engaged during a normal 737 approach, provided the simulated variant, operator procedure and add-on documentation permit it.
APP controls localiser and glideslope capture; it does not choose the landing speed. Select the calculated VREF plus the required wind additive in the MCP, reduce speed as the flaps extend, and verify MCP SPD or the expected FMC speed mode. Our 737 ILS procedure shows how speed management fits around LOC, G/S and APP.
Faithfully modelled NG and MAX aircraft normally command RETARD near the flare when A/T remains engaged, but exact timing and post-touchdown behaviour vary between simulations. If you disconnect autothrottle manually, take positive control of the thrust levers and close them during the flare. Never assume A/T provides complete underspeed, overspeed or configuration protection.