MD-80 Question for those drivers...

jwp_145

GhostRider in the Sky
I was seated on an AA MD-8x the other day, in the seat that has a nice view of the engine nacelle and nothing else!!!!!

So I'm siting there during cruise, and about ten minutes after level-off, the engines started a surge cycle. They would spool up to what sounded like about 75% take-off thrust, and then go back to flight-idle... this oscillation would take about 60 seconds or so.

So I ask the F/O after the flight, and he said it is "because of the operator" and kinda indicated I was bothering him, so I left it at that.

I'm assuming they were JT8D-21x.

Any clue as to why they would go through such a cycle?
 

Derg

New Arizona, Il Duce/Warlord
Staff member
Might have been doing a step-climb or a step-descent.

The autothrottles on the MD-80/DC-9's are electronic drama queens without any semblance of subtlety. If you want to climb, they're going to slam full forward almost, when you level off, it'll pull the power back a little too far and then compensate with a burst of power when you get a few knots below bug speed.
 

Orange Anchor

New Member
As DT noted, the autothrottles on the -80s are not subtle but the F/O may have been offering a dig on the Capt's technique. Some guys use LOTS of thrust for just about anything from taxi to climb. You have the ability with the MadDog to go into a mode where you can adjust the throttles manually and still retain auto-functions. The annunciator will note CLMP indicating the throttles are 'clamped'.

And yes, the -80s all use a variant of the -200 series JT8D.

The -81 used the 209 while the 82 and 8 uses the -217. The 83 and 88 uses the -219 engine. The -90 then changed to the IAE V-2525 with 25,000lb thrust.

The -80 was a fun machine to fly and had many good points. The autothrottles were not one of them.
 

Derg

New Arizona, Il Duce/Warlord
Staff member
It's a world of difference on the 757/767.

In flight, if you ended up a few knows slow or fast, the throttles would articulate pretty aggressively, then create massive waves of power.

On the 757/767, I guess the FMC computes speed trends a little better so you can be a little fast or slow in flight and they won't move at all because in a couple-20 seconds you'll be back on speed.
 

JoelT

Well-Known Member
The auto-throttles on the 717 with the Rolls were pretty awful too. AirTran's landing technique was to leave the autothrottles on till touchdown. I never felt real comfortable the way they would go from full power to idle multiple times down the slope. That includes a hundred feet off the ground.
 

jwp_145

GhostRider in the Sky
Thanks guys, I'm quite sure now that's what was happening, just the autothrottles oscilating around the desired speed.

On a side note, the Cirrus autopilot has servos for the roll control that just simply move that side stick... you can just sit there and watch it oscillate around the desired trim, constantly!
 

Bigey

Well-Known Member
The auto-throttles on the 717 with the Rolls were pretty awful too. AirTran's landing technique was to leave the autothrottles on till touchdown. I never felt real comfortable the way they would go from full power to idle multiple times down the slope. That includes a hundred feet off the ground.
Eeeks i'm not a jet pilot or anything but i thought with jet engines you're supposed to keep them spooled somewhat since they take a while to spool fully up?
 

Jfk-Pilot

Well-Known Member
Eeeks i'm not a jet pilot or anything but i thought with jet engines you're supposed to keep them spooled somewhat since they take a while to spool fully up?

yea i heard the same, i believe i read somewhere that it takes anywhere from 6 to 11 seconds to go from idle to full thrust.
 

launchpad

Well-Known Member
Eeeks i'm not a jet pilot or anything but i thought with jet engines you're supposed to keep them spooled somewhat since they take a while to spool fully up?
This is a big factor when you're configured for landing. If you need to go around, you don't want to wait for the engines to go from 0 to 100...it does take a while. If you've done everything right during the approach, the process of getting configured will get the power up to a point where the lag won't be too bad during a go around.

During decent, the most efficient method is to pull the power to flight idle and manage your airspeed and descent rate with pitch. This results in the best compromise between speed, descent rate, and fuel burn. Since you can anticipate a level off at intermediate altitudes, the spool up lag isn't much of a factor.
 

Orange Anchor

New Member
Eeeks i'm not a jet pilot or anything but i thought with jet engines you're supposed to keep them spooled somewhat since they take a while to spool fully up?
Some engines take longer to spool up than others, especially the big fans. The -200 Series JT8D is not one of them.

On most if not all jet engines, somewhere around 70-80% you get almost instant response to throttle movement. And when on final for landing, you are configured with gear and flaps which creates a significant drag profile that again you are at or above the critical response area on the engine.

Very early jets had a much longer response time and some even had drogue chutes to increase the drag. The B-47 is one I remember. This is NOT a drag chute but is one deployed in flight.



From other sources, the CF-6, a popular big fan, it is about 8 seconds from idle to takeoff thrust. On the CFM-56, the engine on the 737s from the -300 on, it takes about 5 seconds from flight idle to 95%. The engine on early 707s took as long as 14 seconds from idle to T/O thrust.
 

Polar742

All the responsibility none of the authority
Eeeks i'm not a jet pilot or anything but i thought with jet engines you're supposed to keep them spooled somewhat since they take a while to spool fully up?
yea i heard the same, i believe i read somewhere that it takes anywhere from 6 to 11 seconds to go from idle to full thrust.
The cerification requirement for the engines during the landing phase is idle thrust to 95% of rated power in 8 seconds.

Some engines "cheat" by having a flight idle mode, which sets the engines to a higher thrust in the air versus what they would be at on the ground.

Personally, I don't know that I'd wanna trust that to happen close to the ground, especially on a 4-engine wing mounted motor machine. I'm pretty sure the whale would turn 90 degrees off course if one outboard went to full power and the other outboard didn't.....prolly quicker than you could smash the rudder....
 

Dan208B

Well-Known Member
When sitting in the back of an MD-80 or 727, many times the engines are just a bit out of sync. That would produce a sound similar to what you described. Being that the engine sounds cant be heard from the cockpit the pilots dont realize it's driving folks in the very back nuts....
 

jwp_145

GhostRider in the Sky
When sitting in the back of an MD-80 or 727, many times the engines are just a bit out of sync. That would produce a sound similar to what you described. Being that the engine sounds cant be heard from the cockpit the pilots dont realize it's driving folks in the very back nuts....
You are talking about the Wah Wahs... just like tuning a guitar. Totally different than what I heard.
 

Baronman

Well-Known Member
Yeah...even on our RJ where there are no autothrottles, subtlety with power adjustments is lost on many a pilot. Why? I think because you can't hear the engines you don't realize what you're doing....you're just moving the thrust levers back and forth a couple inches. And secondly....Guys chase airspeed like crazy. They totally lose the concept of attitude/instrument flying and just micromanage their airspeed with the thrust levers. The airplane presents the airspeed too well....It's a big magnified number with detail down to the knot.

Here's an example of what I'm talking about. We're given a clearance from FL210 to FL180, the pilot selects the altitude then selects a descent rate of -1500fpm. Right after selecting the vertical speed a large reduction in power is made (but the VSI is only starting to trend downwards). The pilot then notices the airspeed trend vector decreasing and then adds more power than he had originally! Then the airspeed increases drastically as the plane finally begins to pitch down and then the pilot brings the power back again to chase the speed back. The pilot in the end feels good because he kept his speed +/-5 knots and as far as he could tell (or hear) the power was perfect.
 

Orange Anchor

New Member
When sitting in the back of an MD-80 or 727, many times the engines are just a bit out of sync. That would produce a sound similar to what you described. Being that the engine sounds cant be heard from the cockpit the pilots dont realize it's driving folks in the very back nuts....
The problems is often you can synch the fans and the turbines will be out of synch.. or synch the turbines and the fans will be out of synch. No easy answers...
 
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