Fix the PMDG 737 cabin altitude warning in MSFS by checking packs, bleed air, pressurisation mode, outflow valve and take-off horn confusion.
In the PMDG 737 for Microsoft Flight Simulator, the cabin altitude warning sounds when simulated cabin altitude reaches approximately 10,000 ft, usually because the packs lack bleed air, the pressurisation controller is misconfigured, or the outflow valve stays open. Restore the air supply, select AUTO or ALTN, and descend if cabin altitude keeps rising.
What triggers the PMDG 737 cabin altitude warning?
The warning responds to pressure inside the cabin, not the aircraft's indicated altitude. A 737 can cruise far above 30,000 ft while maintaining a cabin altitude safely below the warning threshold. Our explanation of how packs and the outflow valve regulate cabin pressure covers the underlying system logic.
Common causes in the PMDG 737 include:
- No bleed-air source: the engine bleed switches were left off and the APU bleed was switched off after engine start.
- Packs off or tripped: both air-conditioning packs are off, have lost their pneumatic supply, or show a trip indication. One healthy pack can normally pressurise the aircraft, so a single pack being off does not automatically explain the warning.
- Outflow valve open: the pressurisation selector is in MAN, the valve was manually opened, or an automatic controller has failed.
- Incorrect controller setup: the mode selector, FLT ALT, or LAND ALT was not set before departure. An incorrect landing altitude usually causes more trouble during descent than climb, but the complete schedule should still be corrected.
- Open door or simulated failure: an external door remains open, or a pack, bleed system, controller or outflow-valve failure is active in the PMDG failure settings.
A mistake we see constantly after a cold-and-dark start is turning off APU bleed without first restoring both engine bleeds and packs. Compare your flow with our PMDG 737 start-up sequence for the bleed and pack controls.
How do I stop the warning and restore pressurisation?
Silence the horn, stop the climb, verify that pneumatic pressure reaches at least one pack, and make sure the outflow valve is being controlled. If cabin altitude remains uncontrolled, descend rather than waiting for switch changes to take effect.
- Confirm when the horn started. A horn in flight as cabin altitude approaches 10,000 ft is the cabin altitude warning. A horn triggered by advancing the thrust levers on the ground is usually the separate take-off configuration warning described below.
- Silence and assess it. Press ALT HORN CUTOUT, then check cabin altitude, cabin rate, differential pressure and the outflow-valve position indicator. The cut-out only silences the horn; it does not repressurise the aircraft.
- Stop climbing if cabin altitude is rising. In a procedural flight, use crew oxygen and the appropriate non-normal checklist. If pressure cannot be recovered promptly, descend towards 10,000 ft or the minimum safe altitude for the terrain.
- Restore the pneumatic supply. With both engines operating normally, set both engine bleed switches to ON, both pack switches to AUTO, and the isolation valve to AUTO. Confirm that the duct-pressure needles show pressure. A pack cannot pressurise the cabin without a working bleed source; our guide to engine and APU bleed-air sources explains what those indications mean. Do not use APU bleed as a generic high-altitude repair because its pneumatic operation has altitude limits.
- Check the pressurisation controller. Set the mode selector to AUTO, enter the planned cruise altitude under FLT ALT, and set LAND ALT to the destination airfield elevation.
- Use the alternate controller when required. If the AUTO FAIL light is illuminated, select ALTN. If neither automatic mode controls the cabin but bleed air and a pack are available, select MAN and command the outflow valve towards CLOSE in short inputs. Monitor cabin rate and differential pressure rather than closing it and forgetting it. Manual control also requires reopening the valve during descent to avoid landing pressurised.
- Deal with trip indications carefully. A PACK or BLEED TRIP OFF light points to an overheated or failed system rather than a controller setting. Correct the configuration and attempt one deliberate trip reset if appropriate. If the indication returns, leave the affected system off and use the remaining system while descending as necessary.
Even after the air supply is restored, cabin altitude may take time to fall. Watch the cabin-rate indication: a downward trend shows that pressurisation is recovering even if the altitude remains temporarily high.
What do the PMDG 737 indications tell me?
| Observation | Likely cause | Next action |
|---|---|---|
| Both duct-pressure needles are low after APU bleed is switched off | Engine bleeds are off or no pneumatic source is available | Restore the engine bleed supply and verify isolation-valve configuration |
| Duct pressure is present but cabin altitude rises with the outflow valve open | Controller, mode-selection or outflow-valve problem | Try ALTN, then use monitored manual valve control if required |
| AUTO FAIL is illuminated | The active automatic pressurisation controller has failed | Select ALTN and monitor cabin altitude and rate |
| One PACK or BLEED TRIP OFF light is illuminated | Pack or bleed-system trip | Protect the remaining system, stop climbing and troubleshoot the affected side |
| The horn sounds only when take-off thrust is applied | Take-off configuration warning | Retard thrust and correct the take-off configuration |
Why does the same horn sound during take-off?
The 737 uses the same intermittent horn for cabin altitude and an unsafe take-off configuration, but the triggering conditions differ. If it sounds on the runway as thrust is advanced, check that the flaps and leading-edge devices are in a valid take-off position, stabiliser trim is in the green range, and the speedbrake lever is down.
ALT HORN CUTOUT is not the fix for a take-off configuration warning. Retard the thrust levers, correct the indicated configuration item, and run the before-take-off checks again.
What if the cabin altitude warning keeps returning?
If the normal bleed, pack and pressurisation settings are correct but the warning returns, check PMDG's failure controls and any saved panel state. Load a known serviceable state on the ground rather than changing panel states in flight, and verify that no door or system failure has persisted between sessions.
Unexpected system behaviour can also result from an incomplete or mismatched aircraft installation. Use our instructions for updating the PMDG 737 safely before reinstalling or deleting aircraft files.