120.4.
When a pilot plans his fuel load he should take into account various factors that the dispatcher does not usually consider. These include the number of runways available for landing at destination and the possibility of mechanical problems. Unfortunately, it is very difficult to justify extra fuel because one MIGHT have a mechanical or because someone MIGHT block the arrival runway as you roll in on final. Companies do not like spending money on MIGHTS. (Having said that my operation is great, never questioning the captain's decision).
When we decide on our final fuel for a flight it is often what the company suggests because on this day their logic is OK. Oftentimes we see an obvious potential problem and add extra fuel (such as a taxi out for deicing). Many times we accept less fuel than we would like in a perfect world because we are aware of economic pressures and the problem we are concerned about has such low statistical probability of happening. (If you go with the last option you must always have a Plan B, and C, and D).
The third case is when people throw themselves on you mercy. Sometimes the cards turn up wrong. Thank you for understanding that this is not generally stupidity or malice. What happens is that the pilot made a decision based on his best judgement and the information available to him at the planning stage. The game simply changed en-route.
To put the mechanical failure in perspective here are some figures. Our company PLANNED minimum landing fuel is 7000 pounds. MUST land fuel (minimum to fly a very tight VFR patern) is 1400 pounds. If a gear leg fails to lower at the marker you have 5600 ponds of fuel to sort the problem out and get to the threshold. At the fuel burn rate near the ground with the gear partly down this is about 15 mins flight time in which to troubleshhot the problem and fly the aircraft to a new approach - to reach a point where you MUST land on the next approach even if the gear is still partly up.