What is the required altitude to maintain after departing on the Portland Low departure from RWY 18?

The required altitude to maintain after departing on the Portland Low departure from RWY 18 is 500 feet Mean Sea Level (MSL). This altitude is specified to ensure safe clearance over terrain and obstacles in the vicinity of the airport during the initial climb phase. Following this altitude helps pilots comply with procedural requirements and ensures a smooth departure while avoiding any potential conflicts with other airspace users.

Maintaining 500 feet allows for adequate separation from the ground, particularly in controlled airspace near busy airports, while also balancing safety and efficiency in the climb out. Thus, pilots departing from RWY 18 on this specific departure procedure are instructed to maintain this altitude until further guidance or clearance is received from air traffic control.