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Loran
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Staggered Engine Spool-up On Older Aircraft - Why?

Tue Jun 11, 2013 10:37 am

Hi all,

I've been wondering for a while why on some older types the engines are spooled up separately. Here is an example I recorded about 3 years ago, looks like standard operating procedure:

+ Ilyushin Il-18D departing IEV: http://www.youtube.com/watch?v=8ALGqYTjFog

Further examples:
+ Ilyushin Il-62M departing PEK: http://www.youtube.com/watch?v=i7ynGxsYe-4
+ Ilyushin Il-62M departing FNJ: http://www.youtube.com/watch?v=JHep7n94Qbs -> this one is interesting because the entire spool-up sequence takes 45 seconds! Other aircraft would be airborne twice by then.
+ Boeing 707-300 arriving MHD: http://www.youtube.com/watch?v=5dpFLssaSXw -> recorded by a fellow a.net user while I had a middle seat. Even on reverse thrust you can hear a separate spool-up

Not sure what technical reasons there are, possibly it has to do with the manual monitoring of the engine parameters? Any insight would be appreciated. Thanks for your comments.

Regards,
Loran
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tb727
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RE: Staggered Engine Spool-up On Older Aircraft - Why?

Tue Jun 11, 2013 1:50 pm

Quoting Loran (Thread starter):
Not sure what technical reasons there are, possibly it has to do with the manual monitoring of the engine parameters?

It's most likely that they are cable driven and also how "tight" they might be rigged, it may also be the fact that the fuel control unit is mechanical. A lot of the time there just seems to be a little more delay on one engine compared to the others. I flew plane last week that #1 lagged way behind the 2 and 3 on initial throttle up and the week before on another airplane, #3 lagged. Part of the reason on takeoff, we push the throttles up to 1.4 EPR, then they all are stable and we push them up from there, they usually stick together once they come off idle.
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747classic
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RE: Staggered Engine Spool-up On Older Aircraft - Why?

Tue Jun 11, 2013 2:21 pm

In the late seventies or early eighties one of our 747 crew rotations in the far east contained a positioning flight from CMB to KHI by an Aeroflot IL62M.

It was a flight to remember.

Our positioning crew consisted of Captain, F/O, Flight Engineer (myself), Purser, Assistant purser and 11 cabin crew for a full pax 747-206.
By shear coincidence our purser was employed, before joining our airline, with the Dutch intelligence service and was very well able to understand and speak Russian.
After boarding, I asked (via our purser) for a jump seat during T/O or landing in the cockpit and the Aeroflot captain welcomed us (our captain and me ) in the very roomy cockpit, occupied by 5 crew members.
As it turned out, I was the lucky one for the starting, taxi and T/O and our captain was invited in the cockpit before the landing.

The Il62M cockpit crew consisted of Captain (only Russian speaking, very poor English), F/O (only Russian), flight engineer (only Russian), navigator (only Russian) and the RT operator (this was the one in charge, speaking very good english).

- Starting of the four Soloviev D30KU engines was as usual with our airline.
- Also Taxi -out was normal.
- Before line up at the runway the RT operator asked the tower for a 2-3 minute run up time.
- The power levers were very slowly symmetrically (two by two) advanced to the derated T/O setting.
- All engines were allowed to operate about one minute at derated T/O thrust, then the brakes were released.

After the T/O I asked about this procedure . The flight engineer answered in Russian : "our engines are not so good in transient conditions and to avoid overheat problems, we always allow for a thermal stabilization time."
Then he smiled an said : "But these engines are a lot better then the old Kutznetsov engines, installed at the older IL62 types.

Later on, during cruise, we talked further with the cockpit crew and it turned out that these engines were operating very close to the stall line during engine acceleration and deceleration and sometimes the mechanical fuel control could not fully cope with that.
The whole conversation was very nice , because both parties were eager to get some information about our current aircraft types. They asked a lot of questions about our 747's and we -in turn - were very interested in their procedures.


However, after landing, it turned out that our purser discovered during flight, that no passenger oxygen system was installed and we were flying higher than FL310.
As a conseq. this was the very last positioning flight with the IL62M, after contacting our 747 Chief-Pilot.

[Edited 2013-06-11 07:40:16]
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Jetlagged
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RE: Staggered Engine Spool-up On Older Aircraft - Why?

Tue Jun 11, 2013 7:44 pm

On the 707 landing video, all four thrust levers would have gone into reverse together. You can see the reverser cowlings move simultaneously. So any difference is in how the engines respond. The later spool up is probably due to one engine's surge bleed valves closing later meaning the engine will be slower to accelerate. That could be to do with cable rigging or differences in the bleed valves.

With the Russian aircraft it clearly wasn't just a case of rigging differences. The IL-18 video is interesting. I wondered whether they wanted to get the outboards at takeoff power while airspeed was still low, then advance th einboards with less risk of asymmetry. If they advanced all four throttles together and one of the outboards was slow to respond they might get into directional control problems close to Vmca.

One western aircraft I know of where one engine is deliberately accelerated slower than others was Concorde, where engine 4 was automatically N1 limited on takeoff below 60 knots.
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fr8mech
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RE: Staggered Engine Spool-up On Older Aircraft - Why?

Tue Jun 11, 2013 8:52 pm

On the older jets, you stagger the engine acceleration to take-off power in order to assure yourself that the engine is operating normally and take-off power is correctly set.

A whole bunch of things are happening in the engine when it accelerates from idle. Bleed valves move, stator vanes move, pressures and temperatures increase, etc. and all this information is fed back to the fuel control. On the older engines, this fuel control is hydro-mechanical. It takes a bit of time (can be less than a second for some parameters, more than a couple for some others) for it to understand what the engine is doing in relation to the input command.

The old Russian (Soviet?) stuff was a bit cruder and the materials used in the engine weren't as advanced as the engines produced in the West at the same time. So, it took longer for the engines to stabilize, both in power and temperature.

The JT9-7XX series was always a pain in the ass during acceleration on the ground. When we (maintenance) had to run the engines at TO power on the ground, our acceleration time from idle to bleed-shift (the point where the stator vanes are wide open and the 3.0 bleed ring is fully closed and the 3.5 bleeds slam shut) was something like 45 seconds. The engine became fairly stable after that and you could go on up to take-off at a somewhat quicker rate. Nothing like today's engines.
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Pihero
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RE: Staggered Engine Spool-up On Older Aircraft - Why?

Tue Jun 11, 2013 10:39 pm

Quoting Loran (Thread starter):
+ Boeing 707-300 arriving MHD: http://www.youtube.com/watch?v=5dpFLssaSXw -> recorded by a fellow a.net user while I had a middle seat. Even on reverse thrust you can hear a separate spool-up
Quoting Jetlagged (Reply 3):
You can see the reverser cowlings move simultaneously. So any difference is in how the engines respond. The later spool up is probably due to one engine's surge bleed valves closing later meaning the engine will be slower to accelerate. That could be to do with cable rigging or differences in the bleed valves.

  
My opinion, too : The reverses were selected at the same time but # 2's acceleration lagged behind # 1 ( watch the blown intake doors which indicate higher air demands) by some 2 seconds.
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Loran
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RE: Staggered Engine Spool-up On Older Aircraft - Why?

Wed Jun 12, 2013 10:57 am

Thanks very much for the interesting comments.

I noted the difference in the cables, firstly on an Iran Air 747 SP where the #1 throttle was set significantly higher than the other three albeit running with similar parameters. Also on this DETA Aviation Il-62M I noted the throttles are not symmetrically:


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Photo © Loran



Regarding the 707 landing video, you are right. It would probably be quite difficult to engage reversers separately unless the flight engineer assists. All jumpseat landings I have been on the PF is usually in a hurry in getting reversers deployed asap although I find the 707 interesting how long it takes from touchdown until the configuration with spoilers and reversers is set.

Quoting Jetlagged (Reply 3):
The IL-18 video is interesting. I wondered whether they wanted to get the outboards at takeoff power while airspeed was still low, then advance th einboards with less risk of asymmetry.

Unfortunately my older camera from 2010 wasn't able to get the outside view into the right light. The aircraft actually has the parking brakes set well after take-off power was set. You can the speed indicator is just left from the artificial horizon of the co-pilot. Here is a different view from the same departure to get a feeling how long this took:

http://www.youtube.com/watch?v=AdWogRM_biM

Regards,
Loran
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fr8mech
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RE: Staggered Engine Spool-up On Older Aircraft - Why?

Wed Jun 12, 2013 3:17 pm

Quoting Loran (Reply 6):
I noted the difference in the cables, firstly on an Iran Air 747 SP where the #1 throttle was set significantly higher than the other three albeit running with similar parameters. Also on this DETA Aviation Il-62M I noted the throttles are not symmetrically:

There are 2 things going on here:

-rigging of the throttle system
-overall wear of the engine and its components

You can adjust the rigging to compensate for wear of components. And, you can change components when they wear beyond the limits of the engine control system. At some point the engine gets changed.

As I recall, the in service limit on a B747-100/200 is a one knob difference from its neighbor(s) and one and half across the throttles. There was also a knob and a half fly-back limit.
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