The use of one-engine-out taxi techniques is on the rise as one means to reduce fuel burn. By shutting down a single engine of the aircraft when it lands, airlines can reduce carbon emissions produced by taxiing by 20 to 40%, and nitrogen oxide emissions by 10 to 30%. In the case of four-engine aircraft, the pilot shuts down two engines immediately after exiting the runway, saving large amounts of fuel. The technique has been adopted in airlines ranging from Iberia Airlines to Air Qatar.
These same techniques have been used in the past, and concerns have been voiced and issues have been raised. For instance, single-engine taxiing necessitates greater jet blast to move the craft forward, generating a strong asymmetric force that could unbalance the aircraft. Thus, Air Qatar refuses to perform such operations at low visibility, at wind speeds of over 25 knots, or when a 180-degree turn or greater is needed to park. Also, the risk of shutdown of key plane functions, including braking, when engines are turned off or on during taxiing has not been addressed.
- Excessive jet blast to achieve wheel un-stick
- Accidental single-engine take-off (unlikely)
- Creation of adverse thermal cycles in engine components
- Failure to develop standard operating procedures (SOP) and checklists to avoid cancelled take-offs and/or malfunctions
- Increased corrosion on aircraft components on the side of the non-running engine/propeller due to absence of propeller propwash as a result of single-engine taxi (inadequate performance of vent systems). Strong asymmetric force generated by greater jet blast from single engine could lead to unbalancing the aircraft. Shutdown of key plane functions when turning engines on and off.
Corroborating sources and comments
ANALYSIS OF EMMISSIONS INVENTORY FOR SINGLE-ENGINE TAXI-OUT” OPERATIONS; http://catsr.ite.gmu.edu/pubs/Kumar_Sherry_Thompson_ICRAT_Env_Final.pdf
Green Taxiing – Strategies for Reduced Surface Emissions
Ioannis Simaiakis, Harshad Khadilkar, Regina Clewlow, R. John Hansman and Tom Reynolds
Massachusetts Institute of Technology
http://www.aviationpros.com/article/10467486/single-engine-taxi-program-will-reduce-emissions-and-save-fuel (Iberia Airlines in Spain has started utilizing a single-engine taxi program, cutting carbon emissions by 20 to 40%. Single-engine taxiing can also reduce emissions of nitrogen oxides by 10 to 30%. Upon landing, the pilot shuts down two engines immediately after exiting the runway, saving between 352 and 524 pounds of fuel [depending on the aircraft model used].)
https://www.aim.gov.qa/eaip/2015-12-10-AIRAC/html/eAIC/eAIC-2015-07-A-en-GB.html (Air Qatar’s single-engine taxi plan. Note in particular when NOT to single-engine taxi – namely, during low visibility, at wind speeds of over 25 knots, and when a 180-degree or more turn is needed to park.)
https://ifalpa.org/store/14POS22%20-%20Engine-out%20taxi.pdf (IFALPA’s take on the issue of “engine-out taxiing”. It advises against mandatory implementation, citing the large amount of training required and the numerous safety risks, including possible inabilities to brake and asymmetric force unbalancing the aircraft.)