At Nairobi (elev 5000'), with a nice, very long 12,000' runway, hot still night, loaded up with 420 people in a B747-136 (Pratt & Whitney JT-9d engines), it used to take a long time to get airborne. I've timed it at 55 seconds and the radio altimeter used to read 50' as you passed over the end of the runway. Then you have Mount Kenya to get around. It was a take-off and departure that required care.