Visit Al's Roboshack and Buy Heli-Pack from Al were missions in Ratchet & Clank. At the end of the mission "Find a new ship", Ratchet and Clank ran into the Novalis Chairman, who gave them an infobot with a commercial for Al's RoboShack by Captain Qwark. Clank deduced Qwark could still be in Metropolis and that they might be able to find him there to warn him of Drek's invasion.
A Gadgetron vendor with a new weapon will be available past the tunnel connected to the landing area. The Blaster is a simple but effective weapon for your arsenal, with a high rate of fire, good range, and average damage. However, you will likely only have enough to buy either that or the Pyrocitor (if the latter has not already been bought on Novalis). The Pyrocitor offers an even higher rate of fire, but only works on close range. Nonetheless, the Blaster is recommended due to the blarg encountered. After you finish shopping at the vendor, take the path on the right.
Aside from the occasional minor detour to destroy crates and pick up bolts, this path is fairly linear. The first enemies encountered are robomutts and blarg commanders. The former are fairly an easy target for any weapon, even the OmniWrench, but mainly serve as a distraction while the more dangerous commander fires a relatively slow moving rocket in your direction. The rocket is easily dodged, but the commanders will retreat when you get too close (assuming there is an area to flee to).
Upon nearing the second bridge, a blarg heli-commander will fly in, occupying the only route available. Getting too close will cause him to fire on you with a machine gun, therefore you should use the Blaster and first-person mode (hold down ) and take him down from afar. Though, even without the Blaster, it is possible to simply run past him. Past several more robomutts and commanders, and down an escalator is a lone mine layer, a robot that flees upon spotting you, automatically placing mines behind it. The robot is otherwise harmless and the mines are easy to avoid. A final third bridge and the last few enemies stand in your way of Al's RoboShack.
After the cutscene, purchase the Heli-Pack upgrade for 1,000 bolts to finish the "Buy Heli-Pack from Al" mission.