The key to safe wintertime flying is to know before you go
If the use of engine anti-ice is delayed inadvertently until after encountering icing conditions, it must be assumed that ice has accumulated in the engine inlet throat area.
With winter upon us, it’s appropriate to review flight operations in icing conditions. That means brushing up on systems knowledge and procedures for all anti- and deicing equipment available in the aircraft. It’s also a good time to get reacquainted with the nature and characteristics of icing. More specifically, review the different types of icing, when and where icing can be expected, and how to detect and avoid it.
The recommendations that follow are general in nature and are intended to supplement procedures in the approved Pilots Operating Handbook (POH). Please note that in case of conflicting procedures or requirements, the POH is the controlling document.
As defined in the Twin Commander POH, Section II, Limitations, icing or potential icing conditions should be considered to exist when operating in visible moisture such as clouds, fog, and/or any form of precipitation and at an Outside Air Temperature (OAT) of up to +10 degrees C (+50 degrees F) or colder.
TPE331 engine inlet icing is prevented by directing warm bleed air (P3) to an airspace between the outer surface of the lower engine air inlet throat and the anti-ice shield. P3 airflow is electrically controlled through the anti-ice valve. The upper portion of the air inlet, which is adjacent to the gearbox, is kept free of ice by heat transfer from the hot engine oil to the inlet surface.
Prior to flights in potential icing conditions, an operational check of all applicable anti-ice and deicing systems must be performed in accordance with procedures outlined in the approved POH. Selecting the engine inlet heat (L and R INLET) switches to ON causes indicator lights (labeled L INLET and R INLET) to illuminate and the ITT/EGT (turbine temperature) to rise. Turbine temperature rise is due to the diversion of some air from the compressor section.
Failure of the ITT/EGT to rise could mean that the anti-ice valve did not open or, worse, that the valve had been stuck in the open position prior to selecting engine anti-ice (inlet heat) ON. I say worse because, on the ground, an anti-ice valve that is stuck open at an OAT above 10 degrees C (50 degrees F) could cause engine damage. In either case, when proper operation of the engine anti-ice valve cannot be verified, flights into potential or known icing conditions must be delayed until the discrepancy has been corrected.
The operational ground check of the engine anti-ice system is complete and satisfactory when, after switching the system to OFF, a commensurate drop in EGT/ITT is observed and the indicator light has gone out. (Refer also to Operating Information Letter OI331-15, dated April 30, 1997.*)
As applicable, select IGN OVRD (MANUAL IGN), instead of automatic ignition, during takeoffs and landings on wet or slush-covered runways. (Refer also to Operating Information Letter OI331-11 R3, dated April 25, 1997.*)
Engine inlet anti-ice (inlet heat) should be used during all takeoffs, flights, and landings in actual and potential icing conditions. If the use of engine anti-ice is delayed inadvertently until after encountering icing conditions, it must be assumed that ice has accumulated in the engine inlet throat area. In such instances subsequent application of engine anti-ice heat can cause ice shedding and ice ingestion, which may cause a brief airflow interruption and could result in an engine flameout. Therefore, if ice has formed, the first action prior to de-icing is to manually select continuous ignition—IGN OVERD (MANUAL IGN)—ON for both engines. Then switch the engine inlet heat (L or R INLET) to ON.
However, it is recommended to select inlet heat ON one engine at a time to reduce the risk of a double flameout. The ignition system should remain ON until ice shedding has concluded. Moreover, depending upon the type of ignition system installed and regardless whether operating in textbook icing conditions, engine ignition should be selected to IGN OVERD (MANUAL IGN) any time ice is suspected or is observed to be collecting on the propeller spinners, wing leading edges, or unheated propeller blade areas.
Reportedly, several flameouts have occurred following descents out of icing conditions into warmer air. Remember that under some conditions ice accumulation can be difficult to detect visually. In all cases where a takeoff or landing is to be conducted during or shortly after operating in icing conditions, it is recommended that engine ignition be manually selected to ON.
For more information on the proper use of engine inlet anti-ice and engine ignition systems (duty cycle limitations, etc.) please review the Honeywell Operating Information Letter OI331-11 R3, dated April 25, 1997 for the applicable ignition limitations.*
When operating within design limitations criteria, flying in icing conditions can be as routine and safe as flying a closed visual pattern on a clear and sunny day. What makes flying in icing conditions unsafe, however, is inadequate operational knowledge of all available anti/deicing systems and system limitations or discrepancies, and a lack of understanding of the nature of ice and how it affects aircraft performance and controllability. In other words, staying safe in icing conditions is up to you!
*For a copy of the Operating Information Letters cited in the article, please contact the Honeywell Customer Support Center: (800) 421-2133; (602) 365-2180; Fax (602) 365-3343.
For additional support on these or any other engine operational issues, please contact Helmuth Eggeling at the Pilot Advisor Group in Phoenix, Arizona, at (602) 231-2697; Mobile (602) 363-9316; or e-mail [email protected].
