Development Update 06/27/2025
Engine logic, trim systems, and flight model refinements bring the AFL 737 MAX closer to real-world behavior.
We're thrilled to share an in-depth look at the latest developments in our AFL 737 MAX project for X-Plane 12. While much of our recent work happens beneath the surface, the complexity and authenticity we're building into this aircraft will fundamentally transform your flight simulation experience.
Airfoillabs Team
Strategic Development Focus: Building from the Core
Our development philosophy has evolved to prioritize the aircraft's most critical and complex systems first. We've strategically shifted our focus to the FMS (Flight Management System), autopilot, and navigation infrastructure - the digital nervous system that makes modern aviation possible. This approach ensures that when we implement our vertical navigation (VNAV) capabilities, they'll rest on a rock-solid foundation of interconnected systems that behave the way they do in the real aircraft.
Creating an accurate flight model for the 737 MAX requires an intricate dance between numerous systems. Each component affects the others in ways that might surprise even experienced simmers. Let's explore what we've been building.
Engine Logic: The Heart of Performance
The 737 MAX's engines aren't just about thrust - they're sophisticated systems managed by the EEC (Electronic Engine Control), essentially the FADEC (Full Authority Digital Engine Control) system that acts as the brain of each powerplant.
Thrust Management Architecture
We've developed a comprehensive thrust lever ratio system that accurately models both the CFM LEAP-1B27 and LEAP-1B28 engine variants. In the real aircraft, the throttle position doesn't directly correlate to engine output. Instead, the EEC interprets your throttle position based on numerous factors: altitude, temperature, aircraft configuration, and selected thrust mode. We've implemented custom performance tables for each thrust setting:
Amber Line Thrust Limit: the absolute thrust limit - amber line
Maximum Takeoff Thrust including the BUMP thrust option: the maximum power available for takeoff, typically limited to 5 minutes
TO1/TO2 (Takeoff Derate 1/2): reduced thrust settings that extend engine life while providing adequate takeoff performance
CLB/CLB1/CLB2 (Climb Thrust Settings): optimized power settings for various climb scenarios
MAX CONT (Maximum Continuous): the highest thrust setting available for extended use
GA (Go-Around): specifically calibrated thrust for missed approach scenarios
EEC Operating Modes and Idle Logic
The Electronic Engine Control system we've simulated operates in three distinct modes:
Normal Mode (ON): the EEC has full authority, managing all engine parameters automatically. It prevents exceedances, optimizes fuel flow, and maintains engine health.
Soft Alternate Mode: when certain sensors fail, the EEC reverts to this degraded mode. It uses the last available sensor inputs and maintains most protections, but with reduced optimization. Pilots might notice slightly different throttle response and must monitor parameters more closely.
Hard Alternate Mode: the most degraded state, where the EEC provides minimal protection, but still a significant one. In this mode, the throttle lever position more directly controls fuel flow, similar to older hydro-mechanical systems. Pilots must carefully monitor all engine parameters to prevent damage, especially EGT.
Idle Logic: Four Distinct Personalities
Engine idle isn't just "minimum thrust" - we've implemented four separate idle modes:
Ground Idle: optimized for taxi operations with minimal thrust
Flight Idle: higher idle setting for in-flight operations, ensuring rapid acceleration when needed
Approach Idle: calibrated for stable approach configurations
Icing Idle: elevated idle to maintain engine anti-ice effectiveness and prevent flame-out in icing conditions
EEC Operating Modes Preview
Engine Dynamics: Bringing Metal to Life
Beyond the control logic, we've meticulously modeled the physical relationships between:
N1/N2 correlation (the relationship between fan and core speeds)
Spool-up characteristics that vary with altitude and temperature
Oil pressure and temperature behaviors that change with power settings and time
EGT (Exhaust Gas Temperature) responses that pilots monitor during start-up and high-power operations
Realistic vibration signatures that change with engine wear and operating conditions
Fuel flow calculations that account for altitude, temperature, and thrust setting
Currently, 80% of our manual throttle control implementation is complete, laying the groundwork for the autothrottle system to come.
Elevator Trim System: More Than Just Pitch Control
The 737 MAX's trim system gained worldwide attention due to MCAS (Maneuvering Characteristics Augmentation System), but the underlying trim architecture is a marvel of redundancy and precision.
Manual and Electric Trim Implementation
Our trim system features realistic trim rates that vary based on:
Flap configuration (trim effectiveness decreases at high speeds)
Manual trimming - the system knows when you're fighting it
Automated trim modes for flaps up and down
The manual trim wheels provide tactile feedback through varying resistance based on aerodynamic loads. When you're significantly out of trim at high speed, you'll need more force (which we simulate by trim speed increase or decrease) to turn the wheel, just like the real aircraft.
Automated Trim Functions (In Development)
We're currently implementing three automated trim systems:
Speed Trim System (STS): automatically trims the aircraft to maintain speed stability during manual flight with autopilot off. It's subtle but crucial for maintaining the aircraft's handling qualities across its entire flight envelope.
Mach Trim: compensates for the aerodynamic pitch changes that occur as the aircraft accelerates through high Mach numbers, preventing the nose-down tendency common in swept-wing jets.
MCAS: the Maneuvering Characteristics Augmentation System that provides automated nose-down trim inputs under very specific high angle-of-attack conditions with flaps up. We're implementing this system with all the updates and safeguards introduced after the MAX's return to service.
Trim System in Action
Flight Model Refinement and Visual Enhancements
Flight Model Refinement: The Foundation of Realism
Our clean-configuration flight model is approaching real-world accuracy. What we are working on now is the effect of flaps and slats:
Lift curves that accurately represent the MAX's flaps/slats
Drag coefficients across the entire speed range for flaps
Pitching moments that change with configuration and CG position with flaps
Next, we're expanding this model to include:
Spoiler effectiveness variations with airspeed and deployment angle / ground and flight spoilers logic
Landing gear drag and pitching moment contributions
Visual Refinements: Enhancing Immersion
While systems take priority, we've also enhanced the visual experience.
Control Stand Animations: levers now move with realistic speed and travel. The throttle levers follow accurate tracks, trim wheels rotate at proper speeds and have an extendable handle for manual trimming. Flap selection lever movement follows the characteristic tracks.
Annunciator System Overhaul: real annunciator lights don't simply switch on and off - they have a characteristic glow-up and fade that our eyes perceive. We've completely reworked our annunciator textures and lighting system to replicate this behavior. When a warning light illuminates, it brightens over several milliseconds, creating the subtle but authentic visual cue pilots rely on.
Annunciator Lighting Preview
The Path Forward
This foundation of engine management, trim systems, and flight model accuracy serves a crucial purpose: enabling realistic autoflight development as soon as possible. Once our VNAV and autopilot systems come online, they'll be controlling an aircraft that behaves very close to the real MAX. This means holds will be flown with proper bank angles, climbs and descents will honor the real aircraft's performance limits, and approaches will require the same energy management as the actual aircraft.
Every parameter we've implemented, from engine spool times to trim rates, feeds into the autoflight system's decision-making. When the autothrottle commands a thrust change, the engines will respond with the same lag and overshoot characteristics as the real thing. When VNAV calculates a descent path, it will use our accurate drag models to determine when to extend spoilers or call for flap deployment.
We understand that flight simmers and aviation enthusiasts crave this level of detail. You're not just looking for an aircraft that flies from A to B - you want to understand and experience the intricate systems that make modern aviation possible. Every switch should have a consequence, every procedure should matter, and every phase of flight should challenge and reward your growing expertise.
We're building something special here, and we can't wait to share more as development continues.
Stay tuned for our next update.
Clear skies,
Juraj, Airfoillabs
AFL 737 MAX is an independently developed add-on for X-Plane, not affiliated with or endorsed by any aircraft manufacturer. Full disclaimer
