Precipitation, freezing fog or blowing snow can all result in engine inlet, compressor core or fan contamination and/or icing. This contamination can occur prior to engine start or during arrival and departure ground operations with engines running. If present during the preflight inspection, engine ice or contamination must be removed prior to engine start. Should engine core or fan ice be suspected after engines are started, the engine manufacturer's procedures for removal must be carried out prior to takeoff.
Pilots must be ready to make the necessary performance corrections if the aircraft flight manual permits takeoff with engine anti-ice ON.
In the case of older engine types considered vulnerable to excessive fan icing during descent, at low thrust settings in moderate or worse icing conditions, ice shedding procedures may be specified for inflight use. If the aircraft flight manual permits it and engine anti-ice is switched ON in flight after an ice build-up has occurred (or is suspected) it is sensible not to select engine anti-ice to ON on all engines at the same time.
The effects of airframe contamination and the potential consequences of failure to properly de-ice prior to takeoff are well documented and are generally well understood within the aviation community. However, the concepts and liabilities of jet engine contamination, inclusive of engine core and fan blade icing, have not had the same degree of exposure and are less well understood.
Blowing snow, precipitation, freezing fog, slush and other ground contaminants or airport snow removal operations can all result in the contamination of jet engine intakes and components. The area of the engine that will be affected is dependent upon the origin and type of contaminant, and whether or not the engines are running at the time of exposure. The potential for damage due to ice ingestion is significant but the more subtle effects of airflow disruption due to ice accretion on compressor and fan blades can also result in loss of thrust, engine damage or flameout.
The engine anti-ice system, as installed on most types, serves solely to prevent ice build up on the air intake opening of the engine nacelle. It does not prevent ice build up in the primary stages of the engine core (compressor) or on the fan blades. Jet engines are most susceptible to the build up of blade ice in conditions of freezing fog or freezing precipitation while the engine is at or near its minimum rotation speed (ground idle). Compressor and fan blades are aerofoils and, due to the affect that they have on the airflow across them, any ice accretion will normally be found on the back side of the blade. This makes the ice difficult to see during a preflight inspection and also inhibits its removal.
Engines cannot be de-iced with glycol based fluids, as is the norm for airframe de-icing, primarily due to the damage that the fluid could cause to the engine and to the potential contamination of the bleed air system. The normal method of de-icing an engine is by using a brush or broom to remove any loose contamination and then warming the affected areas of the engine to melt any ice. This warming may be accomplished by putting the aircraft inside a hanger for a period of time but, more commonly, a directed flow of hot air from an external heat source such as a Herman Nelson unit is used. Applied heat from an external source is much more efficient if purpose made engine inlet and exhaust covers are put in place to retain the warm air within the engine and to keep cold ambient air out.
One of the main problems when using external heat sources is the difficulty of controlling the temperature of the airstream and the potential for damaging some engine components. Emerging technologies such as tempered steam are being developed for engine core and fan deicing applications to address this issue.
Most engine manufacturers also have a recommended "ice shedding" procedure to be carried out if ice buildup on the fan is suspected during ground operations. This procedure will be undertaken with the aircraft stopped with brakes applied and is accomplished by accelerating the engine to a specified N1 (fan rotation speed) for a specified period of time. The procedure is repeated based on a specified time interval until the aircraft is airborne or the icing conditions no longer exist. Some engine manufacturers also specify a similar procedure in the case of abnormal in-flight vibration after periods in descent in icing conditions at low thrust.
The potential effects of engine core, intake or fan icing and/or contamination are numerous. They include:
- Fan blade damage - caused by failure to remove frozen deposits from the engine intake prior to start.
- Fan blade damage - caused by not following ice shedding procedures for ground or in flight operations at an appropriate interval and in particular at any specifically recommended interval. If an excessive amount of ice is allowed to build up on the fan blades at low thrust settings, subsequent application of high thust can result in blade tip damage as the ice is shed.
- Fan blade and engine core damage - caused by failure of or improper use of engine anti-ice. If ice builds up on the intake ring, it is possible that the deposits may detach and be ingested by the engine causing damage which could result in partial loss of thrust or even flameout.
- Compressor stall or surging - may result from disrupted airflow into the compressor due to ice formation on the compressor blades.
- Erroneous flight deck instrument indications - caused by ice-contaminated or ice-damaged engine probes.
In the worst case scenario, many of these effects could lead or contribute to the loss of the airframe.
To protect against engine damage due to contamination by frozen deposits, the following precautions should be taken prior to flight in weather conditions which are, or have recently been, conducive to their accretion within the engine:
- A thorough preflight inspection of the engine inlet is required to establish if any existing contamination is present. Note that the prevailing surface temperature of the components is as important as the prevailing Outside Air Temperature (OAT) and it is possible that snow or ice may have melted due to the heat of the engine and then refrozen at the bottom of the intake or in the case of some turboprop engines in other difficult to see places within the intake.
- Ensure that any engine air intake covers or blanks have been removed.
- Check the intake for presence of contaminants and if it appears that intake blanks or covers were not fitted whilst adverse weather prevailed, ensure that inspection of the intake is especially thorough.
- In the case of a fan jet engine, confirm free rotation of the fan. It only takes a small amount of ice at the bottom of the engine to prevent fan rotation.
- In the case of a fan jet engine, check the fan and (if they are visible) compressor blades for ice. Note that due to the airflow in a running engine, the ice will generally form on the back of the blades.
- If contamination is present, it must be removed prior to engine start. Any loose ice or snow can be removed with a brush or broom. Any frozen contaminate adhering to the intake or to the fan or compressor blades must be removed using approved methods.
Once the engines have been started, the flight crew should use every means available to minimise contaminant buildup prior to takeoff:
- Ensure engine anti ice is selected on.
Use engine anti-ice as per manufacturer's guidelines. Engine anti-ice is normally turned on when the OAT is 10 degrees C or less and visible moisture is present. Pilots must be familiar with the Adverse Weather section of the Operating Manual for their particular type(s).
- Minimise ground time to the extent possible.
- During taxi, increase the distance from the preceding aircraft to avoid having contaminants blown into the engines.
- Perform the manufacturer's prescribed engine ice shedding procedure at any specified interval or as otherwise recommended. Pilots of aircraft that require to run up engines to a certain RPM at regular intervals should consider aircraft behind them; all pilots should be prepared for higher than normal jet eflux from aircraft ahead.
Some engine types have a defined maximum ground exposure time to freezing rain or freezing fog due to core icing concerns. This time is cumulative and, if core de-icing procedures were not accomplished during a ground stop, the exposure time during the inbound taxi must be taken into consideration as part of the total exposure time. If the time limit is exceeded, the aircraft must return to the gate for engine core de-icing procedures.
- The lack of any reliable cockpit indication that fan blade, compressor icing or other intake icing is occurring. Note that ice accumulation on the fan or compressor blades at low power settings is unlikely to cause vibration but may still be significant.
- The inability to see into the engine from the flight deck and the resulting inability to confirm that the engine is ice free.
- The need for flight crews to rely upon assurances from other persons that their aircraft is free of contaminant after treatment at 'off-gate' or ‘remote’ sites.
- Ensure that the engines are contaminant free prior to start.
- Strictly follow the AOM guidelines for ground operations during icing conditions.
- Additional operational advice is contained in EASA SIN 2008-29.
- If published exposure time limits are exceeded or if in doubt about the effectiveness of ice shedding procedures, return to the gate for appropriate inspections.
Whilst aerodromes are generally well equipped to deal with airframe de-icing and anti-icing requirements, the availability of specialised engine de-icing equipment may be quite limited. When conditions that are conducive to the formation of engine core and fan blade icing exist, this deficiency can result in significant departure delays. These departure delays, in turn, may result in the requirement for the airport to restrict arrivals due to the oversaturation of the available parking stands and other aerodrome resources.
Accident and Incident Reports
Accidents and Incidents resulting from engine icing:
On 13 January 1982, an Air Florida Boeing 737-200 took off in daylight from runway 36 at Washington National in moderate snow but then stalled before hitting a bridge and vehicles and continuing into the river below after just one minute of flight killing most of the occupants and some people on the ground. The accident was attributed entirely to a combination of the actions and inactions of the crew in relation to the prevailing adverse weather conditions and, crucially, to the failure to select engine anti ice on which led to over reading of actual engine thrust.
On 27 February 2001, a Loganair SD3-60 lost all power on both engines soon after take off from Edinburgh. An attempt to ditch in the Firth or Forth in rough seas resulted in the break up and sinking of the aircraft and neither pilot survived. The loss of power was attributed to the release of previously accumulated frozen deposits into the engine core when the engine anti icing systems were selected on whilst climbing through 2200 feet. These frozen deposits were considered to have accumulated whilst the aircraft had been parked prior to flight without engine intake blanks fitted.
On 25 November 2004, a MyTravel Airways Airbus A320 departed the side of the runway at Harstad, Norway at a low speed after loss of directional control when thrust was applied for a night take off on a runway with below normal surface friction characteristics. It was found that the crew had failed to follow an SOP designed to ensure that any accumulated fan ice was shed prior to take off and also failed to apply take off thrust as prescribed, thus delaying their appreciation of the uneven thrust produced.
Note: Although the Association of European Airlines (AEA) ceased to exist in 2016, the most recent of their publications still contain some pertinent information. Readers are cautioned to validate the recommendations of these guidebook using more current information sources.
- The NASA “Pilots Guide to Ground Icing” which reviews the problems caused by ground icing, when ground icing is likely to be encountered, the basics about aircraft de/anti-icing fluids and, in general, how to de-ice and anti-ice an aircraft.