B777GE engine hum sound on start
 

Does anyone know the technical reason for the little hum that the GE90 does as the start cycle finishes and transitions to idle?

https://youtube.com/shorts/GrFplXgfOUw?si=6sOy1p1EguvZNjJY

Right at the end of this video you can hear it.

Off schedule variable stators and bleed valves adjusting?

During start, the GE90 produces some interesting noises associated with the transitions between the various configurations of the DAC (Dual Annular Combustor)(one ring vs. both rings).
They simplified the schedule before EIS, but I recall witnessing an engine start during development and the sound was not unlike a car going through the gears.

The BMW/RR engines on the 717 (MD-95) make interesting noises. I had not realized that in a few hundred hours of flight test as my work station was at the front of the cabin. When I sat in the back on an entry into service flight I was fascinated by the harmonic scale they played. First class passengers are denied the experience.

G’day Gents,

I have, over the years, flown several different airliners with all the major engine brands and model of jet engines and some of them buzz or hum from light off to idle and some do not.

Now this is a memory stretch but I recall a tech instructor once explaining the hum/buzz after the engine lights off as an instability in the compressor airflow as the engine accelerates to idle.

Apparently the engine fueling schedule during start is controlled to ensure the engine is accelerating just on the stable side of the surge line and humming. Cross the surge line and you get a compressor stall and all sorts of problems, some engines are fitted with surge bleed valves to relieve compressor pressure during start to prevent a surge. Some engines modulate the engine anti ice valve to achieve the same result.

Once stable idle is achieved the hum/buzz no longer occurs. Of course there are many noises associated with engine start and operation but the above is how it was explained to me many years ago.

Mind you it could all be rubbish as I said it was a long time ago.

Cheers,

BH.

Not so much compressor instability as burner instability - otherwise known as "combustor rumble". While it's true that starting is generally done at the edge of compressor stability, if it actually goes unstable it'll result in a compressor stall (aka 'start stall')(and that's bad...).
During start, the actual pressures in the combustor are relatively low (especially as compared to the ~30 to 1 range during high power operation). This allows the flame front to 'move around' in the combustor which can give that rumbling sound. Now if that flame front instability happened at power, it would quickly fail the combustor due to localized overheating, but during start the temps are pretty low so it's not an issue.

It's really rather amazing some of the things you can hear - even during high power engine operation - if you know what to listen for. At one point in my career, I was responsible for the engine running functional tests (i.e. the engine running stuff they routinely do before the first flight of a new production aircraft). Back in the JT9D days, they needed to run to takeoff power in order to check/set the stator vane and bleed valve schedules, and JT9Ds were not exactly known for their fan and compressor stability (with FADEC, we normally limit high power static running to climb power). While we routinely tried to point the aircraft into the wind, it wasn't always practical to get it straight into the wind - especially due to jet blast concerns at high power. OTOH, running at high power with no forward speed is very challenging for the inlet with a sidewind, and an engine surge due to inlet separation during a ground run could mean a six-figure repair bill to a brand-new engine. The engine run guys had finely tuned hearing - and in the flight deck you could literally hear the sound of an inlet starting to separate - at which the guy controlling the throttle would quickly slam it to idle. I was initially skeptical that one could hear the inlet separate - until I experienced it first-hand. It was a weird, 'sucking' sound and immediately discernable from the normal 130db roar of a JT9D at TO power.

G’day TD,

An aspect of which I was not aware so I’ve learnt something today.

Cheers,
BH. 👍

TD Racer.
Always a good idea to have an experienced hand on the throttle. My first high power run was an eye opener, the reaction speeds of the old guy sitting next to me was astounding. 👍

Talking about noise, the GE90 at 115,000lb thrust vs. the JT8D in the 737-200 at 14,000lb is ~12dB quieter. Taking the difference in thrust into account that’s ~130x less noise per unit thrust, more than two orders of magnitude! GE9X is likely substantially better than that. Quite amazing progress...

I always remember the RB211 early series on the L1011. They sounded like a powerhouse diesel as they accelerated up to idle.
The RB211 on the B747-200 series also had the diesel sound.but somewhat muted. Bloody good engines to handle in flight too. Way ahead of the P&W.

N2 Vibration at engine start

The GE90 engine family experiences "bowed rotor starts". The phenomenon was first reported early in the GE90-76B program, and occurs to varying degrees on a regular basis. A bowed rotor start can be characterized as an indicated N2 vibration during the engine start sequence, frequently associated with felt vibration on the flight deck. It then dissipates approximately 15 to 30 seconds after the engine reaches idle. A bowed rotor start will not result in any significant engine distress nor is it an indication of an impeding engine operational event.

Years ago….but , standing on the ramp listening to a Viscount or Vanguard taxi in or out and start with those 4 RR Darts was a noise to remember.

Many years ago the CF6-50 had to replace the fuel nozzles (due to a cracking issue I believe) and after the new nozzles were installed the engine sounded quite different on start up.

B742 RR starts were always unique I thought either in it or parked next to it you could tell a roller start.

Hot Starts being the weirdly easiest to hear, the great burble humming and then suddenly immediate silence, swiftly followed by fuel control switch cutoff! Good times. 😂

So what is the explanation for this engine sound(P&W) beginning at 3:00?



Or this one at 6:30(RB211):

FuelFlow’s explanation is correct it’s due to the bow rotor. Basically as the engine cools after shutdown there is a sag. On start until the engine reaches a certain temperature there is a rotor bow and associated vibration…

Best and more and most reliable engine that I have ever used.