Engine Restart in Flight: Critical Procedures and Crew Guidance

Engine Restart in Flight: A Systematic Guide for Flight Crews

An engine flameout or failure in flight represents one of the most critical scenarios in commercial and general aviation. While modern multi-engine aircraft are certified to continue safe flight and landing following a single engine failure, the successful restoration of engine power remains a paramount objective. The procedures for in-flight engine restart are highly structured, varying by aircraft type, altitude, and the nature of the failure. This bulletin synthesizes publicly available guidance from aviation authorities and training institutions to outline the core principles, procedures, and crew considerations for managing an in-flight engine restart scenario. FlySafe analysis shows that adherence to standardized procedures and understanding the underlying rationale are fundamental to safe outcomes.

Core Principles and Initial Crew Actions

The decision to attempt an engine restart is not automatic and is governed by the aircraft's Quick Reference Handbook (QRH) and the specific circumstances of the failure. The initial crew response must prioritize aircraft control and configuration before initiating any restart sequence.

According to guidance from the FAA Safety Team, in certain phases of flight such as immediately after takeoff, troubleshooting may be futile and can waste precious time and altitude. Their guidance for single-engine aircraft explicitly advises, "Don’t attempt to restart the engine. That wastes precious time and altitude that might be needed for the emergency procedure," following a failure at or above the minimum turnback altitude. This underscores the principle that securing the aircraft's flight path is always the first priority.

For multi-engine transport aircraft, the QRH typically provides two distinct procedural paths, as noted in Skybrary. One procedure is for an engine failure where a restart may be attempted. A separate, more critical procedure is for engine fire, severe damage, or separation, where the guidance is clear that "there will be no attempt at restart." The crew's initial assessment, therefore, must determine into which category the event falls. Immediate memory items for securing the affected engine (e.g., fuel lever cutoff, ignition off) must be completed before referencing the QRH for the restart procedure.

Airspace status: Not applicable. This is an aircraft-specific emergency procedure. Affected routes: Not applicable. The procedure is executed regardless of route. Recommendation: Flight crews must prioritize "aviate, navigate, communicate." Complete immediate action items for engine failure or shutdown before consulting the QRH for restart guidance. Know the distinction between restartable and non-restartable failure conditions.

Restart Methods: Windmill versus Starter-Assisted

The method available for an in-flight engine restart is primarily a function of altitude and airspeed. There are two primary methods: windmill (airstart) and starter-assisted.

A windmill restart relies on the forward motion of the aircraft to windmill the engine's fan and core, thereby generating sufficient rotation for a light-off when fuel and ignition are introduced. This method is typically viable at higher airspeeds and altitudes. According to analysis in an engine relight document, a windmill relight remains possible at higher altitudes following an all-engine flameout.

A starter-assisted restart utilizes the aircraft's auxiliary power unit (APU) or another power source to drive the engine's starter motor to rotate the core for start. This method is necessary when airspeed is too low to achieve sufficient windmilling rotation. The same source indicates that a starter-assisted relight is typically only available below FL200. The availability of starter cartridges is also a consideration; guidance from Transport Canada (TCCA-002) states the design "should provide the capability for at least two start attempts of each engine."

Recommendation: Crews must be aware of the altitude and airspeed envelopes for each restart method for their specific aircraft type. Attempting a windmill start outside its envelope or delaying a descent to an altitude where starter-assisted start is possible can compromise the success of the procedure.

Monitoring Parameters and the "Hung Start"

Once a restart attempt is initiated, vigilant monitoring of engine parameters is critical. A successful in-flight engine restart is indicated by a predictable sequence of rising parameters. Skybrary lists the key indicators: rising turbine inlet temperature (TIT or EGT), oil pressure, and RPM (N1/N2).

Crews must also "be prepared to terminate the start in the event of overtemperature, lack of oil pressure, or other malfunctions." In most modern aircraft with a Full Authority Digital Engine Control (FADEC), the system will automatically terminate the restart attempt if parameters exceed safe limits. However, crew awareness remains essential.

A particular challenge is the "hung start," where engine temperature rises without a corresponding increase in rotation speed. The TCCA-002 guidance highlights that flight crews "may inadvertently terminate a successful engine start if they do not have adequate evidence the engine start is clearly progressing." The guidance emphasizes that required flight deck indications must provide awareness that the "Engine Restart is Clearly Progressing," typically by showing the engine has reached idle or selected power. Patience is required, as a starter-assisted relight can take up to 2 minutes, according to the engine relight document.

Recommendation: During a restart attempt, crews must monitor the full suite of engine parameters and understand the normal progression timeline. Do not prematurely abort a start that is progressing normally but slowly, as indicated by a steady rise in core speed.

Special Scenario: All-Engine Flameout and Restart Sequencing

An all-engine flameout is an exceedingly rare but extremely serious event. The procedures build upon single-engine restart principles but introduce critical sequencing and power management considerations.

The engine relight document provides specific sequencing guidance. After an unsuccessful 30-second attempt on one engine, crews have two options: immediately attempt to relight the other engine, or wait 30 seconds with the master switch OFF to ventilate the same engine before another attempt. This ventilation period is crucial to clear excess fuel from the engine core, reducing the risk of a hot start or engine fire on the subsequent attempt.

The TCCA-002 guidance addresses performance, stating the required engine power setting during an in-flight restart "should also occur within a maximum of 90 seconds." However, a longer time may be acceptable if safe flight can be continued and the start is "clearly progressing." This highlights the balance between the urgency to restore power and the need to allow the start sequence to complete properly.

Affected routes: Any route where severe weather (e.g., convective activity, extreme icing) or other operational factors could precipitate a multiple-engine flameout scenario. Recommendation: For all-engine flameout scenarios, crews must follow the specific QRH procedure meticulously, paying close attention to attempted start duration, mandatory ventilation periods, and the sequencing of attempts between engines to manage electrical and pneumatic resources.

Post-Restart Procedures and Operational Prevention

A successful engine relight does not conclude the procedure. The crew must manage the restored engine appropriately and integrate it back into the aircraft's propulsion system.

Skybrary notes that for many aircraft, the QRH recommends letting the newly restarted engine warm up at flight idle before advancing the thrust lever, if flight conditions permit. This allows temperatures and pressures to stabilize, minimizing thermal stress on the engine.

The ultimate guidance is to divert to the nearest suitable airport. A restarted engine that flamed out once should be considered potentially unreliable, and its continued operation should not be assumed for the remainder of the flight.

Prevention remains the best strategy. The FAA Safety Team guidance identifies common causal factors for power loss, including "inadequate preflight inspection, fuel system mismanagement and not accomplishing a checklist." It offers a specific preventative measure for fuel-related issues: "setting the fuel selector valve on the fullest tank prior to engine start" and not moving it again until in cruise flight.

Recommendation: After a successful restart, adhere to QRH guidance for engine warm-up at idle. Declare an emergency, and proceed to the nearest suitable airport for a precautionary landing. Comprehensive preflight procedures and systematic fuel management are key preventative measures.

Key Takeaways for Flight Crews and Operators

In-flight engine restart procedures are a blend of standardized checklist execution and informed crew judgment. Based on publicly available NOTAMs and guidance documents, several universal points emerge. The decision to attempt a restart is conditional and secondary to aircraft control. Understanding the difference between windmill and starter-assisted start envelopes is crucial for timely action. Parameter monitoring requires patience to avoid aborting a valid, slow-progressing start. All-engine failure scenarios demand strict adherence to sequencing and ventilation times. Finally, a successful restart mandates a precautionary diversion.

FlySafe Research emphasizes that this analysis is based exclusively on publicly available, independently verifiable data sources published by international aviation authorities and training bodies. This bulletin is intended for informational purposes and should be used in conjunction with official aircraft manuals and operator-specific procedures. For continuous, data-driven analysis of operational safety factors, consult FlySafe's intelligence platform.