Introduction: I actually asked a turboprop pilot I know about this in person a couple of hours ago in the pub but they deigned not to answer. Anyway I don’t remember this from previous turbo prop flights of which I have made gazillions but I may not have been paying attention or have forgotten about this - but I took two flights in the last couple of days in turbo props, one of which was a Saab 340 and one of which was a Dornier 328 (so am reasonably sure it isn’t to do with one a/c tipe) and had this phenomenon on both trips, hence my
Question: In the last minute or two before landing, why does the noise made by the engines of a turboprop considerably change, including getting much louder? I could think of two possible reasons: maybe the propeller pitch has changed (does that effect noise?) or the aircraft has been descending on idle throttle so far and suddenly needs some now that it’s on approach, either purely due to a shallower glide angle, or possibly due to extra drag from gear and flaps.
Bonus question: Before landing today, for some reason the aircraft, which was approaching the final approach (not sure of technical term) at roughly a 40 degree angle went past the runway centreline before turning onto it, which obviously meant that the ground track was basically a hook shape. I know that becuase I could see the runway out of my window. Any suggestions on why that may have been? The visibility was wonderful so this was certainly deliberate; furthermore it was to a not very busy airport so wouldn’t have been about traffic.
Question 1 - blades brought to full pitch. You would have heard the opposite after climb-out when they trimmed for cruise.
Question 2 - Assuming you saw what you thought you saw the PIC overshot the turn from base to final due to crosswinds or margaritas or just poor flying skills.
It’s definitely possible it was deliberately done due to traffic but usually a turn to final is just that. One thing I didn’t ask was the airport. If you were going into PHX then you’re dealing with visibility of 100 to 150 miles. Most other airports have a fraction of that kind of visibility. I tend to think of a turn to final as 10 miles out at the most which is the typical visibility in the midwest. But going into a desert airport you could see the airport WAAAAY farther out than that.
And you probably heard both the engine power reduced and propeller pitch change. One thing to note about propeller noise is your relationship to it. If you’re sitting next to them on the plane you’ll get more of the noise. You can hear the difference in sound volume at an airshow when the plane passes you and you’re in line with the propeller tips. Some prop tips are borderline supersonic and therefore LOUD.
Airport was EGNS and we landed on rwy 26. And I was very close to the prop, I was in the window seat of row 5 in a saab 340 (and on the occasion a few days earlier was in the window seat of row 3 of a dornier 328)
The turn to final was no more than ten miles (can tell from surface features), I would say around three to six.
However looking at a map I’m totally wrong about the 40 degrees thing. It would be more like 20 degrees. Which makes the late turn even harder to understand!
By the way if it helps you reconstruct the flight path it was a Glasgow-Isle of Man flight and from Glasgow we routed to Prestwick (which we overflew directly - the weather was great and I know that area well so I am certain of this) before turning left over Prestwick; then continued on that course until the approach.
Generally, an engine is able to produce more power at higher RPM.
In all but the simplest propeller planes, the RPM is controlled not by the engine power setting, but by a mechanical governor that adjusts the pitch of the blades to maintain a fixed RPM regardless of the power setting.
Propeller planes operate at low RPM in cruise flight for efficiency and noise abatement, and only operate at high RPM when full power is needed.
Full power is generally needed for takeoff, and when aborting an attempted landing. In order to ensure that they have full power available immediately if they need it in the event of an aborted landing, pilots will increase the propeller RPM before landing. Note that this is usually done without increasing the engine power output at all - the pilot simply adjusts the governor setting, and the governor decreases the blade pitch until the blades have stabilized at the desired higher RPM.