Amphibious aircraft flight crew reported after takeoff they were unable to increase airspeed and climb out of ground effect due to downdrafts and mechanical turbulence from buildings. The flight crew elected to fly under a bridge to build airspeed and were then able to initiate climb safely.
Synopsis
Amphibious aircraft flight crew reported after takeoff they were unable to increase airspeed and climb out of ground effect due to downdrafts and mechanical turbulence from buildings. The flight crew elected to fly under a bridge to build airspeed and were then able to initiate climb safely.
Narrative
Yesterday; while serving as the First Officer in Aircraft X; my Captain and I departed ZZZ with a full plane bound for ZZZ1.We verified weight and balance before departure and ensured that we were within both takeoff weight and CG limitations. Departing the dock we taxied around on the river looking for a good takeoff lane with both favorable winds and decreased wake and boat traffic. Although the winds were coming from the west when we elected to land in Location X upon our arrival to the river; it was determined the winds were now coming from the west south west. We watched another seaplane from our same company; successfully takeoff and depart the river via the same southerly route immediately before us. We determined this southbound takeoff to be the safest and decided to taxi towards the east side of the river; near Location X; to begin our takeoff roll due to the westerly wind component. We verified that we had more than enough distance for a safe takeoff and when we had a good window with decreased boat traffic; began our takeoff run. The plan was to get on step with the assistance of the wind; and gradually turn southbound. We expected there to be downdrafts due to the conditions; however; we anticipated that once we began to fly away we would encounter ascending air as is normally the case. The takeoff began very smoothly in a normal fashion and we were quickly able to get on step. At 50 KIAS; we introduced full flaps to unstick the floats from the water; which is a very normal technique that is used. We were off the water with more than enough time to climb out safely on a normal day with standard conditions; however; when we became airborne today (despite max torque) we were unable to increase speed above 75 KIAS. We were also unable to climb out of ground effect above approximately 20-50 feet. The gusty conditions interacting with the large buildings of the city led to downdrafts and mechanical turbulence that kept sucking us down as we tried to climb. The waters below became too rough for us to safely land due to winds and boat traffic; and despite multiple attempts to climb above the bridge; we were unable to gain any altitude. We attempted to fly level along the river to build airspeed but were unable. We did not have the altitude or conditions to attempt a 180 degree turn northbound on the river. There are large buildings to either side of the bridge so steering around the bridge was not an option either. The only safe option was to continue under the bridge and attempt to either land prior to the other bridge or gain airspeed for a climb out. We cleared the bridge both vertically and horizontally by a very large margin; and once we were beyond; our performance improved drastically (likely due to far less wind obstruction from the city). We were then able to climb out and continue exiting the city southbound and complete the flight to ZZZ1 safely and with no further issues. Passengers were thankful and appreciative of the decision making to get them to their destination safely.
Second reporter narrative
Following a landing in Location X due to the strongly prevailing westerly winds; after having loaded X passengers and some baggage; we left the ZZZ dock preparing for departure.At that time; a stiff breeze (estimated at 20Kts) from the WSW was blowing across the River compounded with outgoing tide movement. OAT gage indicated 33degC.A company amphibian; which had just departed the dock in front of me; elected for a southerly take-off (I was unable to see its path as we were docking). Having analyzed the environmental situation I concluded that a southbound departure appeared to be the safest option; but because of the strong westerly wind component; I judged that taxiing all the way across the river to start the roll close to Location X was necessary. North of Location X towards the bridge some mechanical swell was observed; ruling out taxiing farther north .The idea was to get the plane on the step; helped by the wind; then to progressively turn southbound during the planning phase; with crosswind and a minor headwind component. The tide should also have contributed a little bit to get airborne faster. Descending winds were expected after lift-off; but as we would have been flying away and turning slowly southeast; we would reencounter ascending or at least 'neutral' air(as is typical in this situation); allowing for altitude gain before crossing the bridge close to the west pylon again. This technique maximizes the climbing distance available while permitting an approach to the bridge at an angle (like a ridge).The plane got on step normally and we started turning crosswind. At 50 Kts with MTOP at 2400ft/trq; full flaps were used to unstick. We got airborne about 2/3 of the river width approximately abeam the town; which usually gives enough distance for a safe climb. After lift-off the plane sluggishly started to gain altitude (overall the take-off roll wasn't longer than usual). At 20-50 ft AGL the plane stopped climbing and the airspeed started decreasing. Lowering the pitch did not return any airspeed gain; and we kept porpoising from then on with the airspeed varying from 60 to 75 Kts; while the plane did not climb at all; or climbed very sluggishly.I carefully flew the plane away; aiming for the east shore; but there I only obtained a very marginal altitude gain while we still couldn't reach a safe speed. Landing straight ahead was no longer possible as the river was now very rough; and we were experiencing a tailwind. I then pointed the plane for the middle of the bridge as all the other options had been exhausted. I could not get the plane to climb above the street level of the bridge; nor to accelerate past 75Kts. I advised [the] SIC we needed to go under the bridge. I established a positive descent but still got no airspeed in return. Full power and descending; airspeed never got us above 70-75 Kts. I aimed for a height under the bridge which safely cleared the fin; and mentally prepared for either an emergency landing on the other side; or as an alternative for a crossing beneath the following bridges. Exiting the bridge I considered landing there; but the water looked rough. We were still full power with the airspeed stubbornly below 75 Kts and unable to retract the flaps. I prepared to cross under the other bridge but it felt like we were finally climbing; although slowly. I assessed that I could cross the other bridge safely and I continued climbing; although still unable to retract the flaps. We finally managed to reach 85-90 Kts and a normal climb profile could be resumed after approximately 2 minutes since we took-off.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.