APPENIDX P. USING ICING RATE TO DOCUMENT ICING EXPOSURES P.1 BACKGROUND.
Applicants have always needed to document the characteristics or quality of flight test icing events, besides documenting the MVD, LWC, temperature, and event time. Without previous guidance, applicants or icing data analysts devised various schemes. Some of these schemes are inaccurate, and none of them are legitimate or satisfactory. To promote a sound, useful, and understandable way to document icing exposures, you may use the method described in paragraph P.2 of this appendix.
You may use an icing rate sensor during icing flight tests. These instruments are small, inexpensive, and easy to use. Aircraft may have a related ice detector as standard equipment.
P.2 DATA FROM AN ICING RATE SENSOR.
To show the use of icing rate for documentation purposes, we will use data from a Goodrich model 871FA ice detector as an example. This model is used as an ice detector. It provides an analog signal from which the user can calculate an icing rate. This model provides a voltage signal that usually increases from 1 to 5 volts as ice builds up on a ¼-inch-diameter sensing rod. The signal is 1 volt when the rod is ice-free. The signal increases to 5 volts when about 0.5 mm of ice has built up on the rod. At 5 volts, the heater is energized for a few seconds to deice the rod. The ice build up resumes and starts a new cycle when the rod cools again to below 0qC.
A trace of this signal, as shown in figure P-1, gives clear evidence of when icing occurred during the flight – three main events and two brief ones. A close-up view of the signal (figure P-2) shows the detailed characteristics of the signal and shows how the icing rate is calculated for each of the cycles. An average icing rate over each of the icing intervals seen in figure P-1 is enough to document the icing rate for the interval.
Figure P-1. Example of the Analog Output Volt age From a Model 871FA Ice Detector During an Icing Encounter.
P-1
APPENDIX P. USING ICING RATE TO DOCUMENT ICING EXPOSURES (CONTINUED) The record in figure P-1 clearly shows when icing events occurred. The number of up-and down cycles for each unit time gives an idea of the icing intensity on the time-compressed display shown. You can calculate precise values of the icing intensity from the individual cycles when recorded at one-second intervals, or when displayed on an expanded time scale, as in figure P-2.
Figure P-2. High Resolution Example of Anal og Output Voltage From an 871FA Ice Detector.
The Model 871FA icing detector normally has a sensitivity of 0.5 mm of ice over a 4-volt range (about 1-5 v), the ice accretion rate in mm/min is given by Rate= 7.5 x dV/dt (Eqn. P-1.)
dV/dt is the ratio of the voltage increase (dV, in volts) over a time interval (dt, in seconds) during the ice build up phase of an individual cycle. The conversion factor is 7.5. You can calculate the icing rate if you record the voltage signal every second, as in figure P-2. The icing rates calculated for the ice build up phase of cycles 1-3 in this example are 3.1, 2.1, and 1.9 mm/min, respectively.
P.3 CONVERTING ICING RATE TO ICING INTENSITY.
Applicants may convert the icing rate (mm/min) provided by the ice detector to familiar icing intensity terms, as shown in table P-1. For example, a rate of 1 mm/min suggests a moderate icing rate on the ¼-inch sensing rod. These intensities apply at any airspeed. All that matters is the rate of accretion.
P-2
APPENDIX P. USING ICING RATE TO DOCUMENT ICING EXPOSURES (CONTINUED) Table P-1. Measurable Definitions of Icing Intensity1 Intensity Rate (mm/min) Equivalent Rate Light 0.1 to 0.4 ¼ to 1 inch/hour Moderate 0.4 to 1.3 1 to 3 inches/hour Heavy > 1.3 > 3 inches/hour As explained in References P1 and P2, these rates apply universally to any aircraft component. If the wing is collecting ice at the rate of 0.9 mm/min (2 inches per hour), then that is a moderate icing rate for that wing. A different aircraft at a different airspeed may collect ice at a different rate in the same icing conditions.
The rate measured by the icing rate sensor will be different from the rate of ice build up on the wing. This is because of the difference between the sizes of the wing leading edge and the ice detector sensor, and therefore different water collection efficiencies. You may estimate the icing rate and intensity on the wing from the rate provided by the icing rate sensor. You can do this by using the ratio of the collection efficiencies.
Ratewing = (ßwing/ßsensor)xRatesensor (Eqn. P-2.)
Where the peak local collection efficiencies, ß, can be calculated by LEWICE or other ice accretion computer codes (Reference P3). At a ratio of ß wing/ßsensor = (0.3/0.9) = 0.33, the wing will collect ice at a third of the rate on the icing detector.
P.4 DATA SHEET FOR DOCUMENTING ICING EXPOSURES.
Table P-2 suggests a format for documenting icing events by icing rates and intensities.
The top half of the form lists relevant information about the test aircraft and the icing conditions. The lower half of the form contains details of the individual icing events, as follows:
x Columns A-D identify the selected icing intervals from figure P-1.
x Column F lists the average icing rates calculated as shown in figure P-2.
1 This scale has been proposed as part of a revision to the official icing intensity definitions promulgated in the Aeronautical Information Manual (AIM). The revisions have been recommended by an interagency working group assembled in response to Task 1-B of the FAA Inflight Aircraft Icing Plan of 1997.
P-3
APPENDIX P. USING ICING RATE TO DOCUMENT ICING EXPOSURES (CONTINUED) x Column G lists the equivalent icing intensities on the ¼-inch diameter sensor, using table P-1.
x Columns I and J are the estimated rates and intensities for the aircraft component of interest – in this case the leading edge of the outer wing.
x Column K contains the cumulative product of Rate x Duration, as provided by columns I and C.
x Columns P and Q track use of the IPS.
x Columns S-W document the performance and handling qualities of the aircraft experienced during the icing events. The numerical entries in these columns are based on the level-of-effects scale shown in table P-3.
x Use columns Y-AA for documenting other data, such as data from an LWC meter, a drop-sizing instrument, or drop-impactor slide.
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APPENDIX P. USING ICING RATE TO DOCUMENT ICING EXPOSURES (CONTINUED) Table P-2. Suggested Data Sheet for Documenting Icing Exposures
BCDE G HI JKUM NOPQESTUVWHY ZAA Icing Exposure Record for Flight Tests (hypothetical numbers inserted for illustrative purposes.)
Aircraft model & S/N: B-210, S/N=1416 Component(s) of interest: _outer wing LE Type of ice protection system: boots Date of Flight Test: Nov. 12, 2007 Flight Number.
Location: westem Kansas General cloud types) and conditions: deep, widespread stratus & low ceiling (cloud base = 1000 Ft AGL. @ 2C, cloud tops = 4200 Ft AGL @ -12C) Height of freezing level (AGL.): sfc to 500 _(teet) Starting with an ice-free aircraft (and no descent into warm air for de-icing) 17 - Exposure On 871FA Icing Rate Meter On Component of Interest** Other Conditions Auxiliary Data 18 Start-End Duration Distance Average Equivalent Average Equivalent Cumulative - --- - Average 19] Time Icing rate Intensity" Icing rate Intensity" Ice Depth LWC MVD Engine 20 No. (hhmm:) (min.) (nmi) (mm/min) | (T,L,M,H) (mm/min) (T,L,M,H) (mm) (FL, ASL) deg C. (kt) exposure exposure Speed Power Climb Control Vibration (g/m3) (um) | Pr or EPR 21 22 1 1430 - 1441 11 28 0.7 M 0.35 7500 __-111502 1 untested 1 23 24 2 1455 - 1459 10 0.4 0.2 4.7 7400 -10 125 26 3 1502 - 1514 12 30 0.2 0.1 TAL _ 59 7000 27 28 4 1514-1521 7 18 0.5 L-M 9.4 7500 -11 29 Sums: 34 Notes:
*Icing intensity scale is that proposed by FAA Icing Plan Task-1B working group (2001). (T= <0.1 mm/min, L = 0.1 - 0.4 mm/min, M = 0.4 - 1.3 mm/min, H = >1.3 mm/min) ** These amounts are merely approximations for estimation and comparion purposes only. They are computed from the 871FA data simply by using the ratio of the collection efficiencies.
They give a rough idea of what may accum late if no IPS is active on the component of interest.
*** Level-of-effect scale is that proposed by FAA Icing Plan Task-1B working group (2001). (See separate Table of Effects with definitions.)
Although table P-3 is used for aircraft having approved IPS, it may be used to report effects of icing events on any aircraft.
The data in table P-2 show that the outer wing of the test airplane experienced mostly light icing for 34 minutes (86 nm). After the end of the combined 86 nm exposure, only the power and climb performance showed any significant (level 2) degradation. This degradation occurred even though the boots cycled several times during the event. If the boots had not been used, an estimated 9.4 mm (3/8 inch) of ice would have built up on the outer wing.
The data sheet in table P-2 is an acceptable way to document icing events for natural icing flights, icing wind tunnels, or spray tankers. If an icing rate sensor is not available, you should use the data sheet as much as possible. Reference P-4 provides other acceptable ways of graphing and comparing icing-related effects.
P-5
APPENDIX P. USING ICING RATE TO DOCUMENT ICING EXPOSURES (CONTINUED) Table P-3. Effects on Aircraft2 Aircraft Speed Power Climb Control Vibration Effect (AE) (Note a) (Note b) (Note c) (Note d) (Note e) Level 1 Less than Less than 10% No effect or No effect No effect 10 knot loss increase less than 10% required loss Level 2 10-19 knot 10%-19% 10%-19% loss No effect No effect loss increase rate of climb required Level 3 20-39 knot 20%-39% 20% or more Unusually slow or Controls may have loss increase loss rate of sensitive response slight vibration required climb from control input Level 4 40 or more Not able to Not able to Little or no response May have intense knot loss maintain speed climb to control input buffet and/or vibration Notes:
a. SPEED: Loss of speed because of aircraft icing. This is based on the airspeed before the buildup of ice on the aircraft. This is also before applying added power to keep original airspeed.
b. POWER: Added power required to keep aircraft speed and performance that was being flown before the buildup of ice on the aircraft. Refers to primary power settings, that is, torque, rpm, or manifold pressure.
c. CLIMB: Estimated decay in rate of climb (ROC) due to aircraft icing. For example, 10 percent loss in ROC, 20 percent loss in ROC, or not able to climb at normal climb speed with maximum climb power applied.
d. CONTROL: Effect of icing on the aircraft’s response to control inputs.
Levels 1 and 2. No noticeable effect on response to control input.
Level 3. Aircraft is slow to respond to control input. Aircraft may feel sluggish or very sensitive in one or more axes.
Level 4. Little or no response to control input. Controls may feel unusually heavy or unusually light.
e. VIBRATION/BUFFET: May be felt as a general airframe buffet or sensed through the flight controls. It is not intended to refer to unusual propeller vibration (for airplanes so equipped) in icing conditions.
2 The Task 1-B working group devised this table under the 1997 FAA Aircraft Icing Plan. It was developed for use with pilot reports (PIREPs) for icing conditions, but it is also suitable as a checklist for icing test flights. The table lists four increasingly worsening levels of effects due to icing conditions on three performance factors (speed, power, and climb capability) and two handling aspects (control and vibration). See reference P2.
P-6
APPENDIX P. USING ICING RATE TO DOCUMENT ICING EXPOSURES (CONTINUED) Table P-4 complements table P-2 by presenting a checklist for you to evaluate systems and engines during icing events. You may delete nonapplicable items or add items for a particular design.
Table P-4. Checklist of Items to be Evaluated in Icing Conditions.
Aircraft S/N: Flight No.: Date:
Encounter: 1 2 3 4 Comments Crew visibility Air data Airspeed Altitude Alternate static air Temperature Stall warning operation Autopilot operation Electrical load Engine operation:
Normal air or mode Alternate air or mode Engine cooling Propeller IPS operation Engine inlet IPS operation Ice detection Ice detector operation AFM ice detection (day) AFM ice detection (night) Ice Accretions:
Protected surfaces? (specify) Radome?
Spinner?
Inlets? (list) Side windows?
Nacelles?
Pitot probe?
emperature probe?
AOA probe or vanes?
SW sensor?
Ice detector probe?
Antennae (list) Instrumentation? (list) Aft of IPS?
Other? (list) Ice Shedding:
Propeller IPS normal operation IPS delayed activation Radome Antennae Avionics:
Magnetic heading deviation Com/Nav Landing gear operation Flap operation APU operation Other (RAT, T/R, etc.)
P-7
APPENDIX P. USING ICING RATE TO DOCUMENT ICING EXPOSURES (CONTINUED) P.5 REFERENCES.
P1. “A Workable, Aircraft-Specific Icing Severity Scheme,” R. Jeck, reprint No.
AIAA-98-0094, 36th Aerospace Sciences Meeting (1998), American Institute of Aeronautics and Astronautics, Reston, Va. 20191-4344.
P2. “A History and Interpretation of Aircraft Icing Intensity Definitions and FAA Rules for Operating in Icing Conditions,” Richard K. Jeck, FAA Technical Report DOT/FAA/AR-01/91 (2001), FAA William J. Hughes Technical Center, Atlantic City International Airport, N.J. 08405.
P3. “Ice Accretion and Droplet Impingement Codes,” SAE ARP5903.
P4. “Icing Design Envelopes (14 CFR-25, 29, Appendix C) Converted to a Distance- Based Format,” Richard K. Jeck, FAA Technical Report DOT/FAA/AR-00/30 (2000), FAA William J. Hughes Technical Center, Atlantic City International Airport, N.J. 08405.
P-8
APPENIDX Q. ICE SHEDDING Q.1 ICE SHEDDING.
Applicants must give attention to the ingestion of ice shed from protected and unprotected aircraft surfaces into the engine(s) (14 CFR §§ 23.901(d)(2), 23.903, 23.1093, 25.903, 25.1093,
27.1093, and 29.1093). Evaluate the trajectories and energy of shed-ice to ensure safe flight of the aircraft. This includes the integr ity of the airframe and the operation of the engines and other systems. During or after an icing event, shed-ice may create a hazard by entering engine inlet ducts. This may cause damage or affect the operability of the engine. Shed-ice may also strike and damage other parts of the aircraft or block moving control surfaces. The aircraft design should consider these hazards and take suitable steps to prevent unwanted buildup and release of large pieces of ice.
Maximum ice shedding usually occurs after an ice event when the aircraft flies into temperatures above freezing. Ice may shed from wing and empennage leading edges, windshields, fuselage nose, pitot masts, antennae, propellers, or rotors, for example. Experience indicates that small turbine engines are more sensitive to compressor blade damage and adverse engine operation during ice ingestion than are the larger turbine engines. Ice shedding that impacts aircraft instrumentation may also adversely affect other airplane systems, for example, AOA vanes and pitot systems.
An analytical assessment of the shed-ice trajectories is difficult since the trajectories are influenced by local and downstream flow conditions; the shape, size, and lift-to-drag ratio of the ice fragment; and ice fragment tumbling. Reference Q1 contains information on the use of impingement and ice accretion codes for estimating shed-ice trajectories. Ice shedding is analyzed conservatively since an analytical model of the entire shedding process is not currently available. This compensates for the inexact nature of the analysis.
The role of drop impingement and ice accretion codes in ice shedding studies has been to predict the ice shape, size, and mass before the shedding event occurs. Drop impingement and ice accretion codes cannot predict the frequency of shedding, the shedding event itself, the breakup of ice as it sheds, or the trajectory of shed fragments. The trajectory calculations of drop impingement and ice accretion codes apply only to small particles that are not influenced significantly by gravity. In contrast, shed-ice fragments are large enough to experience gravitational effects and are highly influenced by aerodynamic forces dependent on their shape, orientation, and rotation. If you only evaluate ice shedding by analysis, then a damage analysis should account for the most critical ice shapes that will shed and impact the area of concern.
Many variables (for example, ice fragment shape and mass, aircraft attitude and altitude, airspeed, the airflow around the aircraft, and the manner in which the ice detaches) affect the path of ice shed from the aircraft. Therefore, you may have difficulty showing that shed ice will not enter engine inlet ducts or strike and damage aircraft components. Installing anti-icing protection in critical areas is a desirable approach for resolving an “ice shedding” problem.
Q-1
APPENIDX Q. ICE SHEDDING (CONTINUED) You should understand shed-ice trajectories as much as possible before flight-testing. You should also understand the differences between shed-ice trajectories resulting from tanker tests and those from natural icing conditions before accepting tanker testing for evaluating shed-ice trajectories. Reference Q2 provides added information about the use of icing tankers for evaluating shed-ice trajectories.
Ice ingested by the engines should not interfere with the engines’ operation and thrust. The ice protection of airframe surfaces whose ice buildup affects the integrity of the engines should be considered part of the engine’s IPS. Structural damage analyses may be required if ice shed from forward aircraft surfaces can strike downstream surfaces. Also consider shed-ice damage of aircraft components (including control surfaces and their horns, hinges, control cables, and spoilers) and load-bearing structure.
If you do not install anti-icing protection on critical ice shedding surfaces, then you should conduct an investigation to show that ice shed from these surfaces will not cause an unsafe condition. Conduct “ice shedding” investigations during and after icing events. You should evaluate enough events in all intended operational conditions to ensure there is no hazard associated with ice shedding. Besides the usual measurements and observations made during icing tests, we suggest the following instrumentation or observations:
x Motion pictures to record the trajectory of ice released from the aircraft;
x A data acquisition system for turbine-engine-powered aircraft to record exhaust gas temperature, engine pressure ratio, and rpm to detect adverse effects on engine operation; or x Visual examination of the aircraft for damage before and after ice events, especially near the engine compressor and inlet.
In addition, a damage analysis should consider that the most critical ice shapes will shed and impact the areas of concern.
Q.2 REFERENCES.
Q1. “Ice Accretion and Droplet Impingement Codes,” SAE ARP5903.
Q2. “Airborne Icing Tankers,” SAE ARP5904.
Q-2
APPENIDX R. ICE SHAPES R.1 PROTECTED SURFACES.
R.1.1 Preactivation Ice Roughness.
When showing safe flight in icing conditions, applicants should consider the ice accretion roughness on unprotected and protected aircraft surfaces that occurs before the IPS is effective.
Section 8.2.1.2.b, of this AC discusses preactivation ice. This surface roughness may seem small, but the resulting adverse aerodynamic effects may be significant. Also, since safe flight considerations may require adjustments to the uncontaminated aircraft stall protection and other systems, you may need to adjust AFM performance and maneuvering airspeeds of the uncontaminated aircraft (ACs 23.1419-2B, 25.1419-1, 27-1B, and 29-2C). Safe flight considerations for preactivation ice accretion should include acceptable stall warning and safe flying qualities at and above stall warning. Icing conditions event times should include delay times associated with detecting the first buildup of ice, flight crew reaction time to activate the IPS using recommended procedures, and the time required for the IPS to become effective.
Preactivation ice accretion roughness varies with the aircraft’s IPS technology and use as a deicer or anti-icer, the aircraft’s design, and the IPS operating procedures. Therefore, you should determine the preactivation ice roughness for your aircraft and show safe flight before the IPS becomes fully effective. You should base the roughness you select on the time required for the IPS to become fully effective and the icing condition that results in the most critical effects on airplane flying qualities. Figures R-1, R-2, and R-3 are photographs of preactivation ice on the leading edge of a hybrid NACA 23012 2-dimensional icing wind tunnel model. The reader can gauge the sand roughness of the preactivation ice by the grids formed by the one-inch apart chordwise lines and the two 1-1/4 inch spanwise lines along the leading edge of the model.
Castings of the model surface roughness (shown in figures R-1 and R-2) are available from the FAA William J. Hughes Technical Center, Flight Safety Branch. Figure R-1 shows the surface roughness that may build up during the time required for an ice detector to detect ice accretion.
The preactivation time used included 11 seconds for the first deicing cycle of a magnetostrictive type ice detector in maximum continuous icing conditions and an added 30 seconds to allow the pilot to begin recommended anti-icing procedures. You should provide the preactivation ice roughness to the FAA for our acceptance.
R-1