in the description below , the same element is always indicated by the same reference number , and correspondences between elements are indicated by primed reference numbers . the drawings are not to scale ; various elements may be shown in exaggerated or minimized size for clarity of illustration . the description below relates to preferred embodiments as installed on a lifting surface ( here , the wing or the horizontal tail of an airplane ). however , it will be understood that the invention can be installed to protect any airfoil leading edge surface ( e . g . an engine inlet ). in a known hybrid ice - protection system shown in fig1 a - 1c an airfoil generally indicated by reference number 2 has a leading edge region 4 . a high - pressure surface 6 and a low - pressure surface 8 are located immediately aft of the leading edge region . an electro - thermal anti - icer 10 of the running wet type is used to heat the leading edge region , and an electro - thermal de - icer system formed from electro - thermal heating strips 12 , 14 , 16 , 18 , 20 , and 22 is used to de - ice the surfaces 6 and 8 . a controller 24 is used to supply power to the anti - icer 10 and the strips 12 - 22 . fig1 b schematically illustrates what happens in icing conditions before the de - icer is operated . in use , the controller 24 regulates the temperature of the anti - icer 10 to maintain impinging supercooled water droplets above freezing . the water droplets run back onto the surfaces 6 and 8 , where the droplets freeze to form a layer l of ice . ( see fig1 b .) then , the strips 12 - 22 are energized in sequence to melt the bottom of the layer l of ice , thereby debonding the layer l from the surfaces 6 and 8 of the airfoil . first , the strip 12 is energized . the strip 14 is energized next , and then the strips 16 , 18 , 20 , and 22 in the order listed . this order of energization melts the bottom of the layer l of ice from forward to aft , so that the ice can be peeled aft by the airstream and thereby thrown off the airfoil 2 . when the strips 20 and 22 are energized , ice melted during this energization turns to water and runs back aft of the strips 20 and 22 . this runback water refreezes and forms ridges 6 r and 8 r on the surfaces 6 and 8 . as a result , residual ice remains on the airfoil 2 even after ice covering the strips 20 and 22 has been thrown off ( fig1 c ). if the airfoil 2 is a roughness - sensitive wing on an airplane flying for extended periods during icing conditions , the ridges 6 r and 8 r can grow in size and dangerously diminish maximum lift and angle - of - attack margin - to - stall . the ridge 8 r , being on the low - pressure surface 8 , may be of particular concern . in accordance with the preferred embodiment of the invention as illustrated in fig2 the airfoil 2 ′ has a leading edge region 4 ′ and high - pressure and low - pressure surfaces 6 ′ and 8 ′ respectively . an electro - thermal running - wet anti - icer 10 ′ anti - ices the leading edge region 4 ′. a set of de - icer actuators 50 and 52 periodically de - ice the surface 6 ′, and another set of de - icer actuators 54 and 56 periodically de - ice the surface 8 ′. the anti - icer 10 ′ is temperature - controlled by a control unit 24 ′ using a temperature sensor ( not shown ). the control unit 24 ′ also controls the energization of the actuators 50 - 56 . the actuators 50 , 52 , 54 and 56 are all of the same type , and are disclosed and claimed in u . s . pat . no . 5 , 782 , 435 . ( the disclosure of this patent is hereby incorporated in this patent application by reference .) in this type of de - icer structure , the airfoil 2 ′ has a semi - rigid skin 58 that is supported by a substructure 60 . the actuators 50 - 56 are electrical coils that change shape when short bursts of current are directed through them . when energized using such short bursts of current , the actuators 50 - 56 flex the skin 58 and crack off accreted ice that is thicker than a certain limit . experiments have determined that this type of de - icer can remove ice accretions as thin as 0 . 050 inch . although the preferred embodiment of the invention utilizes the actuators 50 - 56 and the semi - rigid skin , this is only preferred . other mechanical de - icer systems ( e . g . a pneumatic system , an electro - mechanical system , an electro - magnetic system , an impulse system ) can be used instead . the anti - icer 10 ′ is advantageously located entirely within the roughness - sensitive zone of the airfoil 2 ′. generally , the anti - icer 10 ′ is located entirely within the zone where supercooled water droplets strike the airfoil ( the “ impingement zone ”). in this way , the anti - icer 10 ′ is kept fully wetted in icing conditions and heat transfer to the water is maximized . advantageously , the temperature of the anti - icer 10 ′ is maintained near just above freezing . this insures that impinging water droplets do not freeze at the leading edge region 4 ′ and that there is no runback aft of the de - icer actuators 50 - 56 . in practice , the number and the location of the de - icer actuators ( e . g . 50 - 56 ) is chosen to meet the particular requirements of the application . for example , if a wing encounters supercooled large droplets as opposed to droplet sizes specified in far part 25 , appendix c , ice can accrete to 12 % chord or more on the leading edge . the fig2 embodiment has four de - icer actuators 50 , 52 , 54 , and 56 to cope with this . the two front actuators 50 and 54 remove ice accretion from supercooled small droplets , and the two rear actuators 52 and 56 remove ice accretion from supercooled large droplets . the running - wet anti - icer need not be of the electro - thermal type . as is shown in fig3 it is alternatively possible for the running - wet anti - icer to be a bleed air system or a hot gas system . the heated air ( bleed air or air heated in the heat exchanger ) is then routed to a piccolo tube 62 ( typically ) and used to heat the leading edge , the flow rate of the air being controlled using a regulator valve ( not shown ). to prevent the air from heating the actuators 50 - 56 , the air is vented to outboard downstream . an icing tunnel test was conducted using a model similar to fig2 but having only front actuators 50 and 54 . outside the area of the running - wet anti - icer 10 ′, the maximum observed thickness of the accreted ice was less than 0 . 050 ″, with an average thickness of 0 . 025 ″. the measured maximum thickness was independent of the duration of exposure to icing conditions , and there was no frozen runback water in unprotected regions . the test showed : a ) roughness aft of the anti - icer 10 ′ could be maintained below 0 . 050 ″. b ) there was no formation of ice ridges , because runback water froze exclusively in the de - iced zone . c ) exposure to icing conditions could be continued indefinitely without overwhelming the system . as a result , an airplane equipped with the preferred embodiment can be kept for extended periods in icing conditions . although a preferred embodiment has been described above , the scope of the invention is limited only by the following claims :