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yaps30
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A330 Throttle In Cruise Question

Sat Jan 03, 2015 10:19 pm

A quick question for the pros...

I was in an A332 today flying transatlantic and over the eastern half of Iceland (not sure if that's relevant, but in case it is...) the engines seemed to go through 30 second cycles of reducing to idle or near idle and then spooling up to a sound I would typically associate with cruise. The cycles lasted maybe 20 minutes and then the egines went back to a relatively constant cruise sound. According to the in-flight map, altitude and groundspeed (fwiw) stayed roughly the same throughout.

It seemed akin to one of those drivers who regulates speed by accelerating for five seconds and then pumping the breaks, over and over.

I don't think this was a figment of my imagination nor have I noticed it before... but I was wondering if any pros could set me straight?

Thanks!
 
rwessel
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RE: A330 Throttle In Cruise Question

Sun Jan 04, 2015 9:16 am

Perhaps the synchronizers* were acting up, and you were hearing the "beat" of the interference noise from the engines? That can be on the order of 30s. Although it’s usually described more as a whaa-whaa-whaa sort of noise.


*And I'm assuming A330s have them.
 
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SAAFNAV
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RE: A330 Throttle In Cruise Question

Sun Jan 04, 2015 12:49 pm

Quoting rwessel (Reply 1):
Perhaps the synchronizers* were acting up, and you were hearing the "beat" of the interference noise from the engines? That can be on the order of 30s. Although it’s usually described more as a whaa-whaa-whaa sort of noise.


*And I'm assuming A330s have them.

I might be wrong, but I didn't know any jet engines have them.
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tb727
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RE: A330 Throttle In Cruise Question

Sun Jan 04, 2015 4:16 pm

Were you flying through any turbulence?
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yaps30
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RE: A330 Throttle In Cruise Question

Sun Jan 04, 2015 7:22 pm

Quoting tb727 (Reply 3):
Were you flying through any turbulence?

No, not really. There did seem to be a pretty strong headwind if the relatively low groundspeed of around 420 mph was any indication, but the ride was pretty smooth.
 
FlyHossD
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RE: A330 Throttle In Cruise Question

Sun Jan 04, 2015 7:32 pm

Sounds somewhat like wave conditions - the flight encountered a lifting wave and to maintain altitude, the autopilot would (slowly) pitch down. That, in turn, would cause and increase in speed, so the auto-throttles would need to reduce the engine rpm. Then once the airspeed was correct, the engines would resume normal power.

And then the cycle repeated.
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yaps30
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RE: A330 Throttle In Cruise Question

Sun Jan 04, 2015 8:03 pm

Quoting FlyHossD (Reply 5):

Sounds somewhat like wave conditions - the flight encountered a lifting wave and to maintain altitude, the autopilot would (slowly) pitch down. That, in turn, would cause and increase in speed, so the auto-throttles would need to reduce the engine rpm. Then once the airspeed was correct, the engines would resume normal power.

And then the cycle repeated.

That makes perfect sense, thanks.

I forgot to mention it above, but pitch seemed to keep cycling between nose-up and nose relatively straight ahead, so that fits as well.

Thanks to all!
 
LH707330
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RE: A330 Throttle In Cruise Question

Mon Jan 05, 2015 6:09 am

I've experienced a similar thing on an A319 heading from MSY-SFO near the Rockies. I'm assuming that in RVSM airspace you're not allowed to deviate very far, but would it be more efficient to have a "soft salt hold" mode where the plane is allowed to go +/-300 feet to let those oscillations self-regulate instead of trying to dampen it with a pulse/glide to maintain altitude and speed?
 
N243NW
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RE: A330 Throttle In Cruise Question

Mon Jan 05, 2015 6:37 am

Quoting SAAFNAV (Reply 2):
I might be wrong, but I didn't know any jet engines have them.

The MD-80 has this feature. Allows you to match EPR of both engines, or synchronize either N1 or N2 speeds. I'm not sure how widely this feature is actually used by pilots, though. Newer aircraft typically have this type of sync built into the engines' FADEC systems and is largely automatic.

Quoting LH707330 (Reply 7):
would it be more efficient to have a "soft salt hold" mode where the plane is allowed to go +/-300 feet to let those oscillations self-regulate instead of trying to dampen it with a pulse/glide to maintain altitude and speed?

Again, the MD-80 has this feature too. It's a vertical mode of the flight guidance system called "PERF". When this is enabled, the aircraft's natural phugoid motion or other atmospheric factors will allow a slight deviation instead of having the airplane seesawing the autothrottles back and forth to maintain an exact altitude and speed. I'm pretty certain, though, that this feature is almost never used by pilots, especially in RVSM airspace.

[Edited 2015-01-04 22:47:09]
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Max Q
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RE: A330 Throttle In Cruise Question

Mon Jan 05, 2015 7:05 am

Quoting N243NW (Reply 8):
Quoting LH707330 (Reply 7):
would it be more efficient to have a "soft salt hold" mode where the plane is allowed to go +/-300 feet to let those oscillations self-regulate instead of trying to dampen it with a pulse/glide to maintain altitude and speed?

Again, the MD-80 has this feature too. It's a vertical mode of the flight guidance system called "PERF". When this is enabled, the aircraft's natural phugoid motion or other atmospheric factors will allow a slight deviation instead of having the airplane seesawing the autothrottles back and forth to maintain an exact altitude and speed. I'm pretty certain, though, that this feature is almost never used by pilots, especially in RVSM airspace.

We used this mode all the time on the MD80 but that was before the days of RVSM, it worked very well !
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glen
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RE: A330 Throttle In Cruise Question

Mon Jan 05, 2015 9:29 am

The A330 has a autothrust soft mode when the aircraft is at cruising altitude. It keeps the speed within a +/- 3 kt range of the target speed.
If the speed deviation from the target becomes too large, the autothrust soft mode disengages. The autothrust transition from soft to basic mode may lead to transient thrust variation.
So if speed deviation becomes greater than 3 kts the autothrust will revert to basic mode and immediately reduce the thurst considerarbly in order to reduce to the "exact" speed. Once the target speed is established, thrust will increase again and the soft mode reengages.
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SAAFNAV
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RE: A330 Throttle In Cruise Question

Mon Jan 05, 2015 3:16 pm

Quoting N243NW (Reply 8):

The MD-80 has this feature. Allows you to match EPR of both engines, or synchronize either N1 or N2 speeds. I'm not sure how widely this feature is actually used by pilots, though. Newer aircraft typically have this type of sync built into the engines' FADEC systems and is largely automatic.

I see. But that is only to match speeds or EPR, not syncrophase them like on prop aircraft, which is how I understood his meaning?
Or do the engines do that?

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Thanks!
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ZBBYLW
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RE: A330 Throttle In Cruise Question

Mon Jan 05, 2015 3:33 pm

Often times as a pilot in can be concerning how close your speed gets to MMO especially when you are flying at a higher cost index. If you encounter any wave conditions you can see the speed start to creep up, uncomfortably so. Any major bump could cause the speed to increase. What you can do to prevent this is to bring the speed back yourself. If you're flying at M0.8 and the speeds starts creeping up if you select a speed of .78/.79 it will force to airplane to bring the speed back and it does it by (fairly aggressively) bringing the power down.
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rwessel
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RE: A330 Throttle In Cruise Question

Tue Jan 06, 2015 12:11 am

Quoting SAAFNAV (Reply 11):
I see. But that is only to match speeds or EPR, not syncrophase them like on prop aircraft, which is how I understood his meaning?
Or do the engines do that?

Matching N1s is effectively the same as syncrophasing props. At that point the fans will be turning at the same rate.
 
FlyHossD
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RE: A330 Throttle In Cruise Question

Tue Jan 06, 2015 2:37 am

Quoting ZBBYLW (Reply 12):
Often times as a pilot in can be concerning how close your speed gets to MMO especially when you are flying at a higher cost index. If you encounter any wave conditions you can see the speed start to creep up, uncomfortably so. Any major bump could cause the speed to increase. What you can do to prevent this is to bring the speed back yourself. If you're flying at M0.8 and the speeds starts creeping up if you select a speed of .78/.79 it will force to airplane to bring the speed back and it does it by (fairly aggressively) bringing the power down.

Very good point about cruising near the "barber pole." Wave and turbulence is also a concern when cruising slow.
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ZBBYLW
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RE: A330 Throttle In Cruise Question

Tue Jan 06, 2015 2:44 am

Quoting FlyHossD (Reply 14):

Generally in the 320 series I don't find we are cruising near the bottom of the envelope except in the 321 when at heavy weights… In that case you are close to the top end and the bottom end… If you're worried about the airspeed cruising up and down in wave, it's best to go down 2 or 4 thousand feet to widen the envelope if you can.
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SAAFNAV
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RE: A330 Throttle In Cruise Question

Tue Jan 06, 2015 4:32 am

Quoting rwessel (Reply 13):

Matching N1s is effectively the same as syncrophasing props. At that point the fans will be turning at the same rate.

True, but the phase angle won't be matched. But since you way more blades, it probably doesn't make such a big difference?
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FlyHossD
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RE: A330 Throttle In Cruise Question

Tue Jan 06, 2015 5:56 am

Quoting ZBBYLW (Reply 15):
Generally in the 320 series I don't find we are cruising near the bottom of the envelope except in the 321 when at heavy weights… In that case you are close to the top end and the bottom end… If you're worried about the airspeed cruising up and down in wave, it's best to go down 2 or 4 thousand feet to widen the envelope if you can.

Spot on.

I can recall a few instances where we couldn't do that (descend) due to traffic. In one very memorable instance, we got rocked pretty good, but fortunately, everyone had been seated for a while. And in this case, we got the over-speed warning (B737-300).
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N243NW
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RE: A330 Throttle In Cruise Question

Thu Jan 08, 2015 5:59 am

Quoting SAAFNAV (Reply 16):
But since you way more blades, it probably doesn't make such a big difference?

   Exactly. It's really a synchronizer as opposed to a synchrophaser, but the latter really isn't necessary with turbine engines.

Quoting Max Q (Reply 9):
We used this mode all the time on the MD80 but that was before the days of RVSM, it worked very well !

Interesting! Good to know.
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wing
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RE: A330 Throttle In Cruise Question

Tue Jan 20, 2015 6:51 am

In my recent flights after reading this post,I went out for my rest period and decided to use the WC before taking my nap.Our 330-200's has the crew rest in front so we use the lavatory at the mid section which is close to the engines.While waiting in line I notice the sound of the engines close to the one you described.It was not turbulent nor there were any troubles in the engine.After my rest when I returned to the cockpit,I checked the engine parameters and there were some differences between the engine vibrations although they were within the limits.These kind of vibrations occur after maintenance works on the fan blades rasped because of minor dameges.Sound is not exactly the engine thrust change sound but it definetely can be differed from usual engine sound.

The Airbus "autothrust" especially in A330 doesn't make too much drastic changes to chase the speed at high altitudes,the Autopilot allows the airplane to gain and loose altitude to keep the speed even in the turbulent conditions.The A321 was glorious on that there were even couple of times we had to disengage the aututhrust to maintain consistent thrust in very turbulent times.
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