In the Mission 'It came from Outer Space', there is a mission 'A Trip to Dreamland', which requires you to fly to Area 51, complete the collection 'Some Like it Hot', and find a Hydraulic Tool.
The Hydraulic Tool is described as drops always; it doesn't, it drops sometimes, and if you use it to click on a UFO, the flag in the mission which says you've found one is unset, so you have to find another.
I found and used two before I'd completed a 'Some like it Hot' collection, and I've now had 6 more flights to Area 51 without getting another Hydraulic Tool to complete the mission.
So don't use the Hydraulic Tool unti you've completed the mission.
Someone else has said that once the mission is complete, the Hydraulic Tool then does drop from every flight, and you can move on to the UFO collection.
The Hydraulic Tool is described as drops always; it doesn't, it drops sometimes, and if you use it to click on a UFO, the flag in the mission which says you've found one is unset, so you have to find another.
I found and used two before I'd completed a 'Some like it Hot' collection, and I've now had 6 more flights to Area 51 without getting another Hydraulic Tool to complete the mission.
So don't use the Hydraulic Tool unti you've completed the mission.
Someone else has said that once the mission is complete, the Hydraulic Tool then does drop from every flight, and you can move on to the UFO collection.