The ADB-S data from the accident flight of N725TH shows a cruising altitude of 600 feet immediately prior a slight climb and then sharp descent to the water below. Why would the pilot would choose to operate the seaplane at such a low altitude?
The answer is multi-faceted. First, the airspace surrounding the flight corridor from Seattle to the San Juan Islands, though the Gulf Islands and up the Inside Passage, is surrounded by active airports such as Paine Field in Everett, Naval Air Station Whidbey Island, Victoria International Airport, Bellingham International Airport, and further north to Vancouver International Airport. Smaller airports also are spread through this are both on the islands and coastal mainland. Thus, in order to avoid a lot of the busier air traffic around airports, and to avoid mandatory transition clearances, flying low and below certain classes of airspace can be a more efficient and safer alternative.
Second, since the more populous land aircraft need to safely fly within engine out glide distance to shore, seaplanes are not so restricted because in an emergency they can land on water. Thus, there should be less air traffic at lower altitudes over water than higher up. Third, weather and lower ceilings may still allow seaplanes to safely fly their routes at a lower altitude while remaining clear of clouds. Fourth, winds may be more favorable at lower altitudes to reduce flight time and fuel consumption. Fifth, of course, is that the experience for seaplane passengers is enhanced by flying closer to the beautiful scenery below.
The FARs establish standards for minimum altitudes for aircraft, when not taking off or landing. N175TH was operating under the more restrictive regulations of Part 135. These regulations are more restrictive because the operator is conducting air taxi/commercial operations for paying passengers. Part 135 sets forth the general VFR altitudes. For day VFR conditions the minimum operating altitude is 500 feet above the surface.[i]
While there is nothing inherently illegal with flying at lower altitudes, from the early stages of pilot flight training, pilots are taught that altitude translates into time when faced with an emergency, especially one involving loss of engine power. The more altitude below means the more time there is to troubleshoot potential problems, recover from unusual attitudes, and to implement emergency procedures.
* Chris Rusing is an airline captain (Airbus 319) and is an aviation accident attorney with Aviation Law Group PS in Seattle. He has been an attorney and pilot for more than 20 years.
[i] 14 CFR 135.203(a)(1)