Stalls in Instrument Meteorological Conditions (IMC)

2.stall-in-imc. Stalls in Instrument Meteorological Conditions (IMC)

A stall occurs when the wing exceeds its critical angle of attack (AOA), causing airflow over the upper surface to separate and lift to decrease abruptly. The stall is a function of AOA, not airspeed — a wing can stall at any airspeed, any attitude, and any power setting. In instrument meteorological conditions (IMC), the absence of outside visual references, combined with potential distractions, icing, turbulence, and unusual attitudes, makes stall recognition and recovery markedly more difficult than in visual conditions.

Factors That Increase Stall Risk in IMC

• Increased load factor in turns. Stall speed increases with the square root of the load factor: V_s(turn) = V_s × √n. A 60° bank level turn imposes 2 G, raising stall speed by approximately 41 percent. Steep turns flown to intercept courses or correct for overshoots in IMC can quickly approach the accelerated stall regime.
• Structural icing. Even small accumulations of ice disrupt airflow, increase weight, and can raise the stall AOA threshold to a value much lower than the published clean stall AOA. Ice-contaminated wings may stall with little or no aerodynamic warning, and tailplane icing can produce a sudden nose-down pitch when flaps are extended.
• Distraction and task saturation. Programming avionics, copying clearances, briefing approaches, and managing communications can divert attention from primary flight instruments, allowing airspeed to decay unnoticed.
• Power changes and configuration changes. Configuration changes during approach (gear, flaps) and abrupt power reductions during level-off or descent transitions are classic precursors to inadvertent stalls.
• Spatial disorientation and unusual attitudes. A nose-high unusual attitude in IMC can decay rapidly into a stall if not promptly recognized and corrected.

Recognition on Instruments

In IMC the pilot does not have a visible horizon attitude cue and must recognize an impending stall through the flight instruments and aerodynamic feel:

• Airspeed indicator decreasing toward published stall speed (Vs0/Vs1) or the bottom of the green/white arc.
• Attitude indicator showing an increasing pitch attitude or unusual nose-high attitude.
• Vertical speed indicator showing a decreasing rate of climb or developing descent despite high pitch.
• Altimeter showing altitude loss inconsistent with commanded pitch.
• Stall warning horn, AOA indicator if installed, airframe buffet, and sluggish, mushy controls.
• Engine instruments when power is at or near idle while pitch remains high.

Recovery Procedure (FAA Standardized Stall Recovery Template)

The FAA-recommended recovery emphasizes that reducing AOA is the priority, even at the expense of altitude:

1. Disconnect autopilot and autothrottle. The autopilot may be masking the stall by trimming progressively nose-up.
2. Pitch nose-down — apply nose-down elevator until stall indications (buffet, warning, AOA) cease. Trim as required to relieve control pressures.
3. Roll wings level using coordinated aileron and rudder, referencing the attitude indicator.
4. Add thrust as needed — apply maximum allowable power smoothly while monitoring pitch (high power can produce a strong nose-up pitching moment).
5. Retract speedbrakes/spoilers if extended.
6. Return to the desired flightpath once a positive rate of climb and safe airspeed are established. Retract flaps/gear on schedule.

Note that adding power alone is not a stall recovery; AOA must be reduced first. In airplanes with significant icing, recovery may require greater nose-down pitch and higher airspeeds before flap retraction.

Example

During an ILS approach in IMC at 90 KIAS with full flaps and gear down, ATC issues a vector requiring a 30° banked turn. Vs0 in this airplane is 55 KIAS clean, level. In a 30° bank, load factor is 1.15 G, and stall speed becomes 55 × √1.15 ≈ 59 KIAS — but with full flaps and ice contamination, effective stall speed may be considerably higher. If the pilot, focused on intercepting the localizer, allows airspeed to decay to 70 KIAS while pitching up to maintain glidepath, an accelerated stall could occur with very little warning.

Prevention Strategies

• Maintain target airspeeds and apply pitch + power memory items for each phase of flight.
• Cross-check primary and supporting instruments continuously; never fixate.
• Use the autopilot to reduce workload but monitor it — be alert for trim runaway or progressive pitch-up.
• Activate anti-ice/de-ice equipment per the AFM, and exit icing conditions promptly.
• Brief and respect minimum maneuvering airspeeds and approach category speed limits.
• Honor the published maneuvering speed (Va) and approach speeds; avoid abrupt control inputs at low speeds.

Because the cues are subtler and the consequences greater, stall awareness in IMC demands disciplined instrument cross-check and immediate, AOA-first recovery action when warning indications appear.