Winter Operations: What To Do
If Anti-Ice Becomes Deice
By Helmuth Eggeling, Honeywell Pilot Advisor
This column is Part 1 of 2 dedicated to TPE331 Winter Operations specifically discussing icing conditions. Part 2 will examine TPE331 operations in very cold OAT (Outside Air Temperature) conditions.
It�s tough to think of flying in icing conditions when the thermometer still indicates more than 100 deg F, although it is already October. Admittedly, I am talking about Phoenix, Arizona. But taking a look at the current lowest freezing level analysis for the entire North American continent (Fig. 1), we may conclude that winter is on its way.
As defined in the Twin Commander Pilots Operating Handbook (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 OAT of up to +10 deg C (+50 deg F) or colder. Consequently, whenever flying in IMC at a typical Twin Commander cruising altitude we should expect icing conditions!
By design, TPE331 engine inlet icing is prevented by directing hot compressor discharge bleed air (P3) to an airspace passage between the outer surface of the lower engine air inlet throat and the anti-ice shield. P3 air flow 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 de-icing systems must be performed in accordance with procedures outlined in the approved POH. As far as the engine anti-ice system check is concerned, 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 it could mean that the valve had been stuck in the open position prior to selecting engine anti-ice (inlet heat) to "ON". It should be noted that a stuck open anti-ice valve on the ground at an OAT above 10 deg C (50 deg F) could cause engine damage due to compressor blades rubbing against the stationary compressor shroud.
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 "OFF", a commensurate drop in EGT/ITT is being observed and the indicator light has gone out (Refer also to Operating Information Letter OI331-15, dated April 30, 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 has been 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 applications 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, that is IGN OVERD (MANUAL IGN) to "ON" on both engines. Then switch the engine inlet heat (L or R INLET) "ON" one engine at a time in order to reduce the risk of a double flameout. The ignition system should remain "ON" until ice shedding has concluded.
Also, select the IGN OVRD (MANUAL IGN) instead of Automatic Ignition, during takeoffs and landings on wet or slush covered runways.
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. Several flameouts have reportedly occurred following descents out of icing conditions into warmer air.
Remember, 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 OI331-11R10, dated October 01, 2012).
For additional support on these or any other engine operational issues, please contact the Pilot Advisor Group in Phoenix, Arizona: Voice phone: USA (602) 231-2697, FAX: USA (602) 231-2380 or contact me via e-mail [email protected].
Discuss this article in the forums...
