Fix FSX Boeing 737 speed hold: arm A/T, engage SPEED, and diagnose Ctrl+R, MCP IAS/Mach, throttle-axis, climb and descent problems.
In the default Boeing 737-800 supplied with FSX and FSX: Steam Edition, the MCP airspeed knob only selects a target; it does not engage speed hold. Arm A/T, press SPEED—or use Ctrl+R—then leave the throttle axis still. If speed drifts, the commanded climb or descent may exceed the autothrottle’s authority.
Why doesn’t the 737-800 MCP airspeed selector do anything?
Because the speed window is a selector, not an engagement switch. Rotating the knob on the Mode Control Panel (MCP) changes the requested airspeed, but that action alone does not tell FSX to move the thrust levers.
Three separate conditions are involved:
- A target is selected in the MCP airspeed window.
- A/T is armed, giving the autothrottle permission to control thrust.
- SPEED mode is engaged, telling the autothrottle to pursue the selected target.
The compact r/flightsim-style answer is set, arm, engage. A mistake we see constantly is setting 250 knots, seeing 250 in the window and assuming speed hold must already be active.
How do you activate speed hold in the FSX Boeing 737-800?
Stabilise the aircraft, select an IAS target, arm the autothrottle and then engage SPEED mode.
- Start from a recoverable flight condition. Use level flight or a modest climb or descent. If the 737 is already close to a stall or overspeed, recover manually before asking the automation to maintain speed.
- Check IAS versus Mach. A normal knots value indicates IAS. If the window shows a decimal value such as
.78, the MCP is in Mach mode; use the IAS/Mach changeover control before entering a target in knots. - Select the target airspeed. Turn the MCP speed selector to the required value. This sets the target only.
- Arm the autothrottle. Move the cockpit A/T ARM switch to its armed position, or use the default
Shift+Rcommand. - Engage speed hold. Press the MCP SPEED button or use
Ctrl+R, FSX’s standard airspeed-hold toggle. - Verify the response. Leave any hardware throttle still and watch the virtual thrust levers and airspeed trend. Near the selected speed, the correction may be small rather than an obvious surge of thrust.
Ctrl+R is a toggle, so pressing it repeatedly can turn speed hold off again. If the cockpit SPEED button works but the key does not, check the control assignment or a possible add-on conflict; our focused FSX: Steam Edition guide covers Ctrl+R, A/T arming and conflicting assignments.
Speed hold is engaged, so why does the 737 still gain or lose speed?
Because autothrottle has limited authority: it can command thrust from idle to the available maximum, but it cannot always overcome an excessive climb, steep descent or high-drag configuration.
| What you see | Likely cause | What to do |
|---|---|---|
| The selected speed changes, but thrust does not | A/T is not armed, or SPEED mode was never engaged | Arm A/T and then press SPEED; turning the selector is not enough |
| Airspeed falls in a climb while thrust is already high | The commanded vertical speed or pitch requires more thrust than is available | Reduce the climb rate or lower the nose and allow the aircraft to regain speed |
| Airspeed rises in a descent with thrust at idle | The descent is too steep for thrust reduction alone to control speed | Use a shallower descent, begin descending earlier or use speedbrake appropriately |
| The virtual thrust levers move and then snap back or hunt | A noisy or recently moved throttle axis is overwriting autothrottle commands | Keep the axis still, recalibrate it and increase its null zone; temporarily disconnecting it can confirm the diagnosis |
| The cockpit levers move but the physical quadrant does not | The hardware throttle is not motorised | No fix is required; physical lever movement is not expected |
| The target appears as a decimal value | Mach mode is selected instead of IAS | Change back to IAS before selecting a target in knots |
Hardware-axis interference is especially deceptive. A non-motorised quadrant does not follow the virtual levers, which is normal, but electrical jitter can keep sending small throttle inputs even when the handle appears stationary. If the mode engages but speed still wanders, follow our diagnosis for autothrottle drift, axis overrides and insufficient thrust.
Does the autopilot master have to be on?
No. In the default FSX 737, autothrottle speed control is separate from the autopilot master, so it can manage thrust while you hand-fly pitch.
Engaging the autopilot does not automatically arm A/T or select SPEED. Vertical modes still affect the result: an aggressive vertical-speed command can make the autopilot pitch up until maximum thrust is reached, after which airspeed falls. During a steep descent, thrust can reach idle while speed continues to rise.
Do not expect thrust-based speed hold to lower the nose or abandon the selected vertical mode. For the relationship between MCP selections, the flight director, autopilot and active modes, see our broader explanation of Boeing 737 autopilot and mode indications.
Default FSX 737-800 and add-on aircraft behave differently
These exact steps apply to the stock Boeing 737-800 in boxed FSX and FSX: Steam Edition. Advanced add-on 737s may replace FSX’s standard automation, ignore Ctrl+R and require their own MCP, autothrottle and flight-management procedures.
In the default aircraft, HDG, NAV and GPS modes control the lateral path; they do not engage speed hold. Add-on aircraft may model LNAV and VNAV, with VNAV coordinating speed, pitch and thrust according to the programmed route and performance data. In that case, the selected-speed window is not proof that MCP SPEED is active—the Flight Mode Annunciator and the add-on’s documentation determine what is controlling the aircraft.
If an add-on follows the route but ignores the expected speed or vertical profile, treat that as a separate mode problem and use our LNAV and VNAV fault-finding checks rather than repeatedly toggling FSX’s standard speed-hold command.