Aircraft Icing FAR 25, Appendix C charts Prof. Dr. Serkan ÖZGEN Dept. Aerospace Engineering, METU Spring 2014
Outline FAR 25 and FAR 29 Appendix C charts Using FAR 25 Appendix C charts Liquid water content as a function of horizontal extent and ambient temperature Liquid water content as a function of horizontal extent and droplet size Alternative ways to document test data and compare with Appendix C Water catch rate (WCR) and total water catch (TWC) Icing severity definitions Variation of icing severity as a function of horizontal extent and ambient temperature Comparing test data with natural probabilities Serkan ÖZGEN 2
FAR 25 and FAR 29 Appendix C charts FAR 25 App. C consists of 6 figures. Has been in use since 1964 for selecting values of icing-related cloud variables for the design of inflight ice protection systems for aircraft. First 3 figures are known as continuous maximum conditions representing stratiform icing conditions or layer-type clouds. The last 3 figures are known as intermittent maximum conditions representing convective or cumuliform clouds and icing conditions. Serkan ÖZGEN 3
FAR 25 and FAR 29 Appendix C charts Traditionally, continuous maximum conditions have been applied to airframe icing protection, Intermittent maximum conditions have been applied to engine ice protection. Figures 1 and 4 indicate the probable maximum (99%) value of cloud water concentration (liquid water content LWC) expected over a specified reference distance for a given temperature and representative droplet size in the cloud. Reference distance: 17.4 nm (20 statute miles) for continuous maximum clouds, Reference distance: 2.6 nm (3 statute miles) for intermittent maximum clouds. Serkan ÖZGEN 4
FAR 25 and FAR 29 Appendix C charts The actual drop size distribution (typically 1-30 microns) in clouds is represented by a single variable droplet median volume diameter (MVD). Overall MVD 15 microns in stratiform clouds, Overall MVD 19 microns in convective clouds. The MVD has proven useful as a simple substitute for the actual droplet size distributions in ice accretion computations. Serkan ÖZGEN 5
Continuous maximum (stratiform) atmospheric icing conditions, Figure 1 Serkan ÖZGEN 6
Continuous maximum (stratiform) atmospheric icing conditions, Figure 2 Serkan ÖZGEN 7
Continuous maximum (stratiform) atmospheric icing conditions, Figure 3 Serkan ÖZGEN 8
Intermittent maximum (cumuliform) atmospheric icing conditions, Figure 4 Serkan ÖZGEN 9
Intermittent maximum (cumuliform) atmospheric icing conditions, Figure 5 Serkan ÖZGEN 10
Intermittent maximum (cumuliform) atmospheric icing conditions, Figure 6 Serkan ÖZGEN 11
Using FAR 25 Appendix C charts There is no comprehensive guide for using, interpretation and application of Appendix C. Design engineers typically select a conventionally recommended MVD and a temperature appropriate to the flight level of concern and use them to obtain the probable LWC from Figure 1 or 4 of Appendix C. Serkan ÖZGEN 12
Using FAR 25 Appendix C charts Selecting exposure distances (HE) LWC values obtained from Figure 1 or 4 are valid only for the reference distances of 17.4 nm or 2.6 nm, respectively. If there is a reason to design for a longer or shorter exposure distance, the LWC originally selected may be reduced or increased by a factor obtained from Figure 3 or 6 in Appendix C. Longer averaging distances will result in lower maximum LWC. Serkan ÖZGEN 13
Using FAR 25 Appendix C charts Selecting exposure distances (HE) Common applications: To estimate ice buildup amounts on unprotected surfaces during a long exposure of 100-200 miles. LWC obtained from Figure 1 is reduced by the factor obtained from Figure 3. To estimate ice buildups on unprotected surfaces during a 45 minute hold. LWC obtained from Figure 1 is used at full value, without reduction. This assumes the worst case in which the holding pattern happens to be entirely within a 17.4 nm region of cloudiness containing the maximum probable LWC. Serkan ÖZGEN 14
Using FAR 25 Appendix C charts Selecting MVD values Common applications: For computing the impingement limits of droplets (chordwise extent of ice accretion) on an airfoil an absolute droplet diameter of 40 microns is used. In general, MVD=20 microns is used for the computation of ice accretion amounts for standard exposure distance (17.4 nm) or longer. Another reference recommends the use of the entire range of MVDs. The designer is advised to consider exposures to droplets with an MVD up to 40 microns over distances up to 17.4 nm at least. Serkan ÖZGEN 15
Using FAR 25 Appendix C charts Difficulties comparing with test data Users often wish to plot the points representing combinations of LWC, MVD and temperature used in Wet wind tunnel tests, Computer simulations, Test flights behind airborne spray tankers, and test flights in natural icing conditions. on Figures 1 and 4. The problem is that these figures are valid only for the fixed averaging distances. A better way is to convert Figures 1 and 4 to equivalent, distance based envelopes where the LWC curves have already been adjusted for the distance effect. Serkan ÖZGEN 16
Continuous maximum LWCs converted to distance adjusted values Serkan ÖZGEN 17
Intermittent maximum LWCs converted to distance adjusted values Serkan ÖZGEN 18
Appendix C curves converted to distance based format (MVD=15 m) Serkan ÖZGEN 19
LWC as a function of HE and T a (Continuous maximum, MVD=15μm) Serkan ÖZGEN 20
LWC as a function of HE and T a (Continuous maximum, MVD=20μm) Serkan ÖZGEN 21
LWC as a function of HE and T a (Continuous maximum, MVD=30μm) Serkan ÖZGEN 22
LWC as a function of HE and T a (Intermittent maximum, MVD=20μm) Serkan ÖZGEN 23
LWC as a function of HE and MVD (Continuous maximum, T a =0 o C) Serkan ÖZGEN 24
LWC as a function of HE and MVD (Intermittent maximum, T a =0 o C) Serkan ÖZGEN 25
The entire supercooled cloud database (660 icing events, 28 000 nm in icing conditions) Serkan ÖZGEN 26
Graphing flight data (t exp =10 min, V =150knot) Serkan ÖZGEN 27
Graphing flight data (t exp =10 min, V =150knot) Serkan ÖZGEN 28
Sample flight data compared with Appendix C Continuous maximum, Appendix C, MVD=15μm Serkan ÖZGEN 29
Sample flight data compared with Appendix C Continuous maximum, Appendix C, T a =0 o C Serkan ÖZGEN 30
Icing tunnel test points on Appendix C envelopes Continuous maximum Serkan ÖZGEN 31
Icing tunnel test points on Appendix C envelopes Continuous maximum, MVD=20μm, V =174 kt Serkan ÖZGEN 32
Water catch rate In some applications, such as in testing thermal antiicing systems, the rate of water catch is important. For a given amount of LWC, the speed at which the aircraft flies through it and the droplet collection efficiency of the wing is important in determining how much heat is required to keep the leading edges at a required elevated temperature. Water catch rate is calculated from: WCR V tot LWC Serkan ÖZGEN 33
Water catch rate Serkan ÖZGEN 34
Total water catch Another item of interest for an icing encounter may be the total amount of ice accreted on certain components, such as unprotected surfaces. Here, the rate of water (ice) accumulation may not be important, but rather the total water catch during the encounter(s). The TWC may be useful for estimating the weight of ice accreted on aircraft components, except for any losses due to shedding or melting. Total water catch is calculated from: TWC tot HE LWC average Serkan ÖZGEN 35
Total water catch Serkan ÖZGEN 36
Acceptable Exposures What is an adequate exposure, or how much exposure is enough? This can be set in terms of TWC. Maximum TWC from the envelopes for a 17.4 nm exposure at the same temperature as the available icing conditions during the test flight can provide a reference. This can be used as the target TWC to be achieved during the test flight. Serkan ÖZGEN 37
Icing severity definitions Test exposures can be reported based on whether the encounters correspond to trace, light, moderate or severe icing conditions. Icing severity can be calculated from: db dt a V r Icing severity Trace Light Moderate Severe Time expired for 0.25 ice formation t > 1 hour 15 min < t < 60 min 5 min < t < 15 min t < 5 min Serkan ÖZGEN 38
Continuous maximum, Appendix C converted to icing severity envelopes Serkan ÖZGEN 39
Sample icing intensity compared with cont. Max., Appendix C, converted to icing severity envelopes Serkan ÖZGEN 40
Comparing test data with natural probabilities The differences between FAR 25 App. C and nature The envelopes in Appendix C do not show all the values that can exist in nature. They also do not give information about the probability of encountering various LWCs, MVDs, temperature durations in icing conditions. Only the probable maximum (99% percentile) values of LWC are shown. Designers of ice protection systems for military aircraft would like to consider lesser percentile values of LWC to accept more risk as a tradeoff against extra weight, space and electrical power reqirements. Serkan ÖZGEN 41
Comparing test data with natural probabilities Flight tests and icing wind tunnel tests Serkan ÖZGEN 42
Natural 99% limits vs altitude for highest temperatures available at the altitude (MVD=15-20 m) Serkan ÖZGEN 43
Natural probabilities for LWC averages at altitudes < 2500 ft AGL Serkan ÖZGEN 44
Natural probabilities for LWC averages at altitudes 5000 ft ± 2500 ft AGL Serkan ÖZGEN 45
Natural probabilities for LWC averages at altitudes 10000 ft ± 2500 ft AGL Serkan ÖZGEN 46
Natural probabilities for LWC averages at altitudes 15000 ft ± 2500 ft AGL Serkan ÖZGEN 47
Natural probabilities for LWC averages at altitudes 20000 ft ± 2500 ft AGL Serkan ÖZGEN 48
Sample flight data compared with natural probabilities for LWC averages at altitudes< 2500 ft Serkan ÖZGEN 49
Natural HE limits and 99% LWC limits for different MVDs in stratiform clouds at 0 o C to -10 o C Serkan ÖZGEN 50
Sample flight data compared with natural 99% LWC limits for different MVDs Serkan ÖZGEN 51