B747 Captain experiences pneumatic leading edge flap-drive failure on approach. Secondary (electric motor) mode successfully extended LED's; followed by a normal landing.

Narrative

[We had a] pneumatic leading edge flap-drive failure. On approach; during flap extension; received FLAPS PRIMARY and LE SINGLE DRIVE EICAS messages. Slight yaw and bank experienced; followed by EICAS display. Captain was able to visually confirm left outboard Leading Edge Devices (LEDs) had not extended. Secondary (electric motor) mode successfully extended LEDs; followed by normal EICAS display; and slight yaw which had been induced was removed. Trailing edge flap operation was normal. Once secondary mode had extended LEDs; remainder of approach was uneventful with normal configuration. After landing; all flaps appeared to retract normally. [We] debriefed with mechanic on arrival. The Irregular Procedure Action items deal with Trailing Edge Flap (TEF) operation in secondary mode. Time for flap operation was not applicable to our situation. Suggest splitting action items into two parts; trailing edge with the present applicable time delays; and leading edge with time required for LED extension (unless accompanied by TEF in secondary mode; once the LEDs are extended the remainder of flap operation will be normal). I do not believe the 160 KT speed restriction is applicable with only a leading edge drive in secondary control mode.