Takeoff in Conditions of Freezing Drizzle and/or Light Freezing Rain (Fixed-Wing Airplanes)—Part II

by Paul Carson, Flight Technical Inspector, Certification and Operational Standards, Standards, Civil Aviation, Transport Canada. This is the second of a two-part article on this critical subject. Part I appeared in Aviation Safety Letter (ASL) 4/2009.

Background

During the winter of 2005–2006, a Transport Canada Civil Aviation (TCCA) inspector observed a number of airplanes operated by various air operators taking off in conditions of freezing drizzle (forecast and actually reported). The inspector considered that the operations were in contradiction of Canadian Aviation Regulations (CARs), specifically CAR 605.30:

De-icing or Anti-icing Equipment

605.30 No person shall conduct a take-off or continue a flight in an aircraft where icing conditions are reported to exist or are forecast to be encountered along the route of flight unless

(a) the pilot-in-command determines that the aircraft is adequately equipped to operate in icing conditions in accordance with the standards of airworthiness under which the type certificate for that aircraft was issued; or

(b) current weather reports or pilot reports indicate that icing conditions no longer exist.

Subsequent discussion identified that air operators and flight crews have insufficient information when faced with conducting a takeoff in these conditions. These discussions also identified that nothing in the current regulations and standards authorizes, nor strictly prohibits, takeoff during conditions of freezing drizzle and/or light freezing rain.

Hazards associated with in-flight operation in supercooled large drop (SLD) icing conditions

Start of contamination
Anti-ice fluids are designed to flow away from the aerofoil critical leading edge region and off the trailing edge as airspeed increases. Although this behaviour will differ for different fluids, different airfoils, different temperatures, etc., a reasonable assumption is that the critical leading edges will be free from all fluid at rotation. Once again, approval of flight in icing conditions includes demonstration of satisfactory performance of the ice protection systems (IPS) as well as demonstration of satisfactory handling qualities and a measurement of the performance degradation with the ice expected on both the unprotected surfaces and any residual ice on the protected surfaces resulting from proper operation of the IPS. Although not just limited to taking off in freezing drizzle and/or light freezing rain, approval also includes other conditions in U.S. Federal Aviation Regulation (FAR) 25, Appendix C, one being the assumption that ice accretion on surfaces begins at liftoff.

Impingement limits
With SLD icing conditions, the droplets are larger and have greater momentum due to the higher mass. The droplets will impact the leading edge of an airfoil section over a greater chord-wise extent than the smaller droplets associated with FAR 25, Appendix C conditions. In addition, SLD droplets may splash and break up into smaller fragments, which may run back prior to freezing. IPS that have been designed to prevent ice build up (anti-icing systems) or remove accreted ice (de-icing systems), have not been demonstrated to be effective in SLD icing conditions.

Pneumatic boot operation in SLD icing
A problem has been identified in the design of pneumatic boot de-icing systems on some airplanes where the chord-wise extent of the boot-protected area did not consider SLD icing conditions, thus resulting in ice accretion aft of the protected area. This accretion has been particularly hazardous when a residual ridge of ice is left just aft of the boot on the upper wing after boot operation to break off ice. Flight tests on several different airplanes, using a tanker airplane to simulate in-flight SLD icing conditions, have shown that a ragged, span-wise ridge forms just aft of the protected area.

Takeoff in Conditions of Freezing Drizzle and/or Light Freezing Rain (Fixed-Wing Airplanes)—Part II, Transport Canada

One effect of this ridge can be non-linear hinge-moment characteristics on trailing edge controls. For unpowered controls, hinge-moment anomalies at the surface can result in pulsing of the pilot’s control, and in the extreme, a reversal in the direction of the pilot’s force can occur. That is, the control can self-deflect to an extreme position, and excessive pilot effort can be required to return the control to a neutral position.

One accident and two incidents in SLD icing conditions

The section below describes one accident and two incidents where encounters with SLD have been documented. There are other encounters that have been documented in various databases where SLD was suspected, but much of the information was collected for other reasons, not specifically for SLD icing conditions.

ATR 72 accident at Roselawn (31 October 1994)
The National Transportation Safety Board (NTSB) in the United States concluded that this accident occurred due to ice accretion on the wing upper surface just aft of the leading edge pneumatic boot and in front of the trailing edge ailerons. The airplane was in autopilot control during a hold at approximately 8 000 ft with the flaps partially extended. The flaps were then retracted. The increase in the wing angle of attack (AOA) due to the flap retraction caused a flow separation at the wing tip due to the ice accretion. The flow separation caused a hinge-moment discontinuity at the aileron, which in turn caused the ailerons to self deflect to full deflection. The autopilot was unable to correct the overbalance and the airplane had a lateral departure from which recovery was not accomplished.

The icing conditions identified in this accident included SLD icing conditions. Much of the aircraft accidents in SLD conditions deal with the arrival phase, long holds at slow airspeeds similar to this accident.

Transportation Safety Board of Canada (TSB), Aviation Investigation Report, Roll Oscillations on Landing, Airbus A321-211, Toronto/Lester B. Pearson International Airport, Ontario, December 7, 2002, Report Number A02O0406

At approximately 16:07 Eastern Standard Time (EST), an Airbus A321-211 airplane was on approach to Toronto/Lester B. Pearson International Airport (LBPIA), Ont., with 123 passengers and 6 crew on board. At approximately 140 ft above ground level (AGL), on final approach to Runway 24R with full flaps selected, the airplane experienced roll oscillations. The flight crew leveled the wings, and the airplane touched down firmly. During the approach, the airplane had accumulated mixed ice on areas of the wing and the leading edge of the horizontal stabilizer that are not protected by anti-ice systems.

Approximately three hours later on the same day, another Airbus A321-211 airplane, with 165 passengers and 7 crew on board was on approach to Runway 24R at LBPIA. At 18:59 EST and approximately 50 ft AGL, the airplane experienced roll oscillations. The flight crew conducted a go-around, changed flap settings, and returned for an uneventful approach and landing. At the gate, it was noted that the airplane had accumulated ice on areas of the wing and the leading edge of the horizontal stabilizer that are not protected by anti-ice systems. There was no damage to the airplane or injury to the crew or passengers.

Given the similarities between these two occurrences, the TSB concluded (1) “It is likely that the icing conditions encountered by both aircraft were outside the Federal Aviation Regulation 25, Appendix C envelopes used for certification of the A321,” and (2) “Drizzle droplet size ranged from 100 to 500 microns. Federal Aviation Regulation (FAR) 25.1419, Appendix C envelope for certification of flight in icing conditions has maximum mean effective drop diameter between 40 and 50 microns.”

The full report can be found at the following Web site: www.tsb.gc.ca/en/reports/air/2002/A02O0406/A02O0406.asp.

Meteorology measurement criteria forecasting/reporting freezing drizzle and/or light freezing rain vs. FAR 25, Appendix C

Weather forecasts are not made in terms of FAR 25, Appendix C parameters such that they would match the certification icing environment. Also, pilot reports (PIREP) of icing conditions are unique to the airplane from which they are reported—light icing to a Boeing 727 could be heavy icing to a Beech Baron.

Appendix C is not adequate for freezing drizzle and/or light freezing rain given that the maximum droplet size in the appendix is 40 microns for stratiform droplets and 50 microns for cumuliform droplets, whereas the smallest probable drizzle droplet size is 100 microns, and raindrops begin at 500 microns. Furthermore, any cumulus cloud that has a vertical extent that is greater than its horizontal base may include “appreciable numbers” of droplets that are larger than 50 microns.

A minor point, but it should be noted that maximum drop size in FAR 25, Appendix C is 40 microns (or 50 microns) median volume diameter (MVD) or mean effective diameter (MED), not absolute diameter droplet size. The “smallest” freezing drizzle and/or light freezing rain drops are actually measured in absolute diameter terms, not MVD or MED.

Transport Canada Civil Aviation Requirements

Icing certification
In general, TCCA follows the same certification requirements as the Federal Aviation Administration (FAA). These requirements include use of FAR 25, Appendix C as a definition of the in-flight icing atmosphere. TCCA does have additional guidance material on how compliance must be demonstrated for performance and handling qualities. This guidance has led to different limitations and/or configurations of IPS for many foreign airplanes, mainly turbopropeller powered airplanes. In some cases, other authorities have subsequently adopted these additional measures after accidents.

Operational requirements in CARs Part VI and Part VII
The relevant operational regulations relating to flight in icing conditions are contained in CARs Part VI—General Operating and Flight Rules and in Part VII—Commercial Air Services. The following extracts are pertinent:

(a) CARs Part VI, Subpart 2—Operating and Flight Rules
602.07 No person shall operate an aircraft unless it is operated in accordance with the operating limitations

(a) set out in the aircraft flight manual, where an aircraft flight manual is required by the applicable standards of airworthiness.

(b) CARs Part VI, Subpart 5—Aircraft Requirements
605.30 No person shall conduct a take-off or continue a flight in an aircraft where icing conditions are reported to exist or are forecast to be encountered along the route of flight unless

(b) current weather reports or pilot reports indicate that icing conditions no longer exist.

It should be noted that there is a proposal to change the content of CAR 605.30 contained in Notice of Proposed Amendment (NPA) 1998-252 to read as follows:

605.30 No person shall conduct a take-off or continue a flight in an aircraft under IFR where icing conditions are reported to exist or are forecast to be encountered along the route of flight or under VFR into known icing conditions unless

(b) current weather reports, pilot reports, or briefing information relied upon by the pilot-in-command indicate that the forecast icing conditions that would otherwise prohibit the flight will not be encountered during the flight because of changed weather conditions since the forecast.

The intent of the proposed change is to permit more flexibility in operating in reported icing conditions. However, it does not clarify the situation with respect to taking off in freezing drizzle and/or light freezing rain. In addition, the present status of the NPA is with the Regulatory Unit (RU) of TCCA pending publication in Canada Gazette, Part 1.

(c) CARs Part VII, Subpart 4—Commuter Operations
704.63(1) No person shall conduct a take-off or continue a flight in an aircraft when icing conditions are reported to exist or are forecast to be encountered along the route to be flown unless the aircraft is equipped to be operated in those conditions and the aircraft type certificate authorizes flight in those conditions.

(d) CARs Part VII, Subpart 5—Airline Operations
705.69(1) is identical to 704.63(1).

Interpretation of operational requirements
As noted above, the aircraft flight manuals (AFM) of currently certified airplanes do not contain any specific limitations prohibiting takeoff in SLD icing conditions. The Type Certificate may or may not reflect the wording in the AFM, but will specify whether the certification basis includes the applicable FAR paragraphs relating to ice protection. Also, the Type Certificate is not a document that is generally familiar to air operators and flight crews. It is possible through a Supplemental Type Certificate (STC) to have an IPS (more frequently seen for small airplanes) added to airplanes that would include additional limitations regarding flight in icing conditions.

The AFMs of some airplanes do contain a limitation indicating that if severe icing conditions occur (as identified by various visual cues), the airplane must immediately exit these icing conditions. Severe icing is noted as including freezing drizzle and/or light freezing rain. The differences in measurement criteria between FAR 25, Appendix C and aviation meteorological reports remain.

Conclusion
TCCA continues to collect and analyze data in consultation with other authorities worldwide in an effort to enhance present day knowledge regarding the safety of flight in conditions of freezing drizzle and/or freezing rain.

References:

J. C. T. Martin, Transport Canada Aircraft Certification Flight Test, Discussion Paper No. 41, The Adverse Effects of Ice on Aeroplane Operation, Issue 2, 4 July 2006.
J. C. T. Martin, Transport Canada Aircraft Certification Flight Test, Discussion Paper No. 50, Takeoff in Conditions of Freezing Drizzle or Freezing Rain (Fixed-Wing Aircraft), Issue 2, 29 September 2006.