Learjet 31 question

[inner]

Hello,

I have a question for the learjet riders. Its about the fuel system.
What i don't understand is that at my schematic scheme i see an engine driven fuel pump with a high pressure line direct going to the jet pump, situated in the wing tank. From the jet pump there is a low fuel pressure line back to the engine.

What i would like to know if this is a closed circuit?
Or is the high pressure fuel line just going to the wing tank?

someone can help me??

[inner]

it's ok, i understand now, scheme was not very clear

[ICT_SLB]

Just as a clarification, the HP line is also known as the "Motor Flow Line" to the main fuel ejector in the wing ( a venturi). The line back to the engine is LP but (relatively) high flow.

[jrsanch]

The beauty of the jet pump operation is that it uses venturi pressure from the high pressure fuel coming from the engine (low volume) to draw or suck the fuel at low pressure (high volume) back to the engine with no moving parts. Very reliable operation with an electric standby pump in case the system fails, which might be due to a foreign object blocking the venturi inside the jet pump. Also, the standby pump is used for engine start and crossfeed or crossflow into the other engine/tank.
Should the high pressure pump within the engine fail, then there is no way that the engine can sustain itself (according to ground school) nor there is enough pressure from the standby pump to keep it running.
This system has been in Lear's since the old 23, through the 30 series, 50 series and 40, 60 series.

[jrsanch]

"Motor Flow Line"
They rather call it "Motive Flow"

[inner]

But if the jet pump fails, can the standby pump pressurize the fuel tank??

[jrsanch]

Learjet is very proud of its system reliability and very few have failed, like I mentioned before, usually due to a foreign object left inside of the tank (rags, etc).
The jetpump does not pressurize the tank, rather the vent system does. The sole function of the jet pump is to feed the engine with high volume, low pressure fuel.

[BelArgUSA]

OK - I have been teaching the Lear 23/24 and 25 systems, many moons ago.
Shall we say nearly... 40 years ago...? Early 1970 era...
And I continue to fly Lears presently, 31A, and 35/36.
I just stay "barely current" on types.
xxx
Here are some facts about the Lear fuel systems -
NB - The Learjets s/n to 23-029 did not have JET PUMPS... (Elec. Pumps only).
xxx
(1)
Jet pump switches (L and R) control the opening/closing of the MFCV...
MFCV = Motive Flow Control Valves, 28VDC, wired (spring loaded) to OPEN position.
IN CASE of 28VDC failure > MFCV fail OPEN (feeding the respective engine).
(2)
IN CASE of a "LOW FUEL PRESS" light ON (less than 2 PSI pressure) -
Select L (or R) Jet Pump OFF, turn SEFP (Standby Electric Fuel Pump) ON...
If in cruise, and this low pressure light is ON, descent to or below FL250.
This will permit RAM AIR to pressurize wing/fuselage tanks to "above .25 PSI".
High VMO suggested to keep pressure.
(3)
What can cause a L (or R) MFCV to close...? -
(a) Related L (or R) Jet Pump Switch OFF (powering MFCV closed), or...
(b) Engine start switch L (or R) to START position.
(4)
There are no moving parts in a jet pump... they "never fail"...
If "motive flow pressure" is present, "Jet Pumps" will be operating.
(5)
In Learjets equipped with TIP TANKS, (23/24, 25 and 35/36), note that -
Bottom 1/2 tip tank fuel NOT available IF related L (or R) MFCV is closed, or tip tank jet pump failed.
(6)
Procedures - L (or R) LOW FUEL PRESS light ON =
(a) select L (or R) Standby Electric Pump ON, and, if light still ON -
(b) select L (or R) Jet Pump OFF...
(7)
L (or R) Fuel Jettison (DUMP) available from TIP TANKS only (if installed).
xxx
The above are all my notes for the Learjet fuel system...
... And stop laughing at me if I flare 30 feet above TDZE... (sorry, old 747 landing habit).
Still dont know why the boss (airline CEO) want ME to fly him...?
By chance I have a F/O more competent than me.
xxx
:)
Happy contrails

[inner]

Hello,

Can somebody explain me a bit the difference between the control wheel trim switch and the control wheel steering switch?

It's a bit confusing for me.

e.g if the a/c is on alt hold mode and i want to change my altitude, i change this with the control wheel trim switch. If autopilot is engaged does it react immediately or do i have to push the cws switch and release to make it react.

Plse explain the difference.
thx

[jrsanch]

I guess it will be all clear when in the sim or the actual airplane. Think of it as using the arming button and not. When flying manually, you use the arming button, when in autopilot, you don't. Instead you move the what Learjet calls the "barrel". If on autopilot, and a change in altitude is desired, move the barrel up or down (a few clicks, the more you push or click the bigger the movement of the control surface), just remember to set altitude preselect.
If flying manual, you push the arming button (aka control wheel trim switch) and move up or down in order to trim the stab.

[aerodog]

An excerpt from "How Bill Lear Built the Model 23"

www.aerotalk.com/index.cfm?action=view&ForumID=15 (http://www.aerotalk.com/index.cfm?action=view&ForumID=15)

Another series of events provide another glimpse into the working of Bill Lear. The early Model 23 aircraft had seven electric fuel pumps. Two in each wing, one in each tip tank and one in the fuselage fuel tank. From day one they had a high failure rate. In spite of the fact they were of new manufacture, the high failure rate was known in the market place and did not help the image.
Electric fuel pumps were Bill's area of expertise, airborne electro-mechanical components. Months went by, all manner of testing took place but no one could figure out why the pumps were failing. Finally on one of his trips to the West Coast he learned of a jet pump system used in military aircraft that had no moving parts. It relied on excess fuel volume produced by the engine driven fuel pump that was routed in a small line to the fuel tank and the jet pump. It would pick up more fuel and return to the engine in a bigger line.
http://www.aerotalk.com/images/jetpump_sm.jpg (http://www.aerotalk.com/ViewImage.cfm?ImageFile=jetpump_lg.jpg&Caption=Schematic of the jet pump.)Bill learned the Ronson Company (yes, the same company that made cigarette lighters for your coffee table) made jet pumps for the military. He contacted them, asked for a jet pump for testing and a quote for a production order. They complied with both requests. The pump worked fine...the price would be about $4,000 each. An outrageous price for an item with no moving parts. It took less than a week for Bill, the engineers and machinist to design a small casting, a screen to keep debris out of the pump, and two standard AN hydraulic fittings, one modified on a lathe to convert it to a nozzle. The cost in production, about $200 each. Four of the seven electric pumps could be replaced by Bill's jet pumps.


Post script to the story:About the time Bill was getting the jet pump designed, somebody figured out why the electric pumps were failing. The brushes were backed by a single spring. There was a resonant frequency in the airplane that would excite the brush...cause it to vibrate...cause it to arc and burn. The fix was simple. Back the brush with a second spring. Now no single resonant frequency could excite the brush. The jet pumps stayed.

[lionelmandrake]

Great article on Lear! It's my favorite airplane, and I always enjoy reading about the history of the developement.
By the way, do you know where I can get any flight test info on the Lear? That would be very interesting stuff I should think. I have done a few stall tests in the 20 and 30 series aircraft for return to service after repair of the wing and/or removal of the leading edge. It's very attention getting to say the least. I also had the joy to fly a 35 with the Raisbeck aft baggage locker. When I got up to altitude and set cruise power as usual, the aircraft went right up to the barber pole! I had to pull it back to almost a hundred pounds per hour less fuel flow (per side!) so I wouldn't overspeed! Quite a mod that.
Keep up the great posts, ciao.

[Flemish]

Hello

Instead of starting a new topic i'm going to ask my question hear:

next month i'm going for the lj35. I want to prepare myself but i only have a lj31 manual. I was wondering if i can use this manual?

Are the systems of both aicraft almost the same or is not worth using the lj 31 manual?

Thx

[BelArgUSA]

Goeiendag Flemish...
xxx
The Lear 31 will give you a general idea of the Lear designs...
But for a 35, you better be aware of fuel system differences.
Fuel system limitations, tip tanks limitations, fuel transfer etc...
xxx
Why in hell not get a 35/36 manual.
If you order one from the factory, specify which serial number.
There are many system differences and options.
For general study, any 35/36 manual can do. Expect "differences"...
xxx
:)
Happy contrails

[Flemish]

The reason i only have the lj31 is because i could download it. But if it gives my a general idea i think i'm already a step ahead for the groundcourse.

[aerodog]

lionelmandrake,

Have no access to any flight test reports, but yes true stalls in the Learjet are a bit sporting and only done with a forward CG location.

BTW, did you know Pedro Larranaga, Learjet's longtime rep in Lima?

[BelArgUSA]

Hola Flemish -
xxx
Ok, TFE 731 engines will be same. You might or might not have reversers.
Fuel (tip tanks) transfer procedures differences.
For fuel, common points are jet pumps, you can study the concept.
Fuselage tank same procedure, but if 36, different transfer procedures.
You might or might not have single point refueling system.
Some 35/36 have fuel dump system, for fuel jettison from tip tanks only.
Electrical - 28V DC part likely to be about same, expect 115V AC differences.
Hydraulic system very similar, as well as brakes. Same emergency brakes.
Same nose wheel steering system, 115V AC...
Pressurization, air conditionning, oxygen system same or similar.
Avionics of all Lears are different, as per customer options.
xxx
My primary activity is training/instruction...
In all honesty, no instructor expects you to "study anything" prior to courses.
Or if they expect you do so, they would send you manuals or familiarization data...
But welcome to the Lear world. They are fun to fly...
My son just got hired as F/O 31A, he got a Lear rating recently. His first job.
All the best to you -
:)
Happy contrails

[Flemish]

Thx, you helped me a lot.
Looking forward to fly this plane.

F

[Flemish]

Before i forget:

How are we going to learn about the FMS: is that just by the book or just in the simulator? Don't have experience of that.
Or does the training provider have a CBT of that?

[BelArgUSA]

For FMS, you better wait for CBT and simulator...
They are all different. Generally Honeywell, but there are others.
You really want to put your instructors to unemployment are you...?
xxx
:8
Happy contrails

[Flemish]

Hahaha
Not al all, but it is my first type and i want to succeed. A golden rule in aviation is 'Good preparation'.

:8

[BelArgUSA]

Niet duwen, zeg meneer a.u.b.
Say, mister. dont push, please...!
:E
Oh I understand you... I know many get "nervous" in your case -
But I often do check-rides for highly experienced pilots...
They fly the simulator (or airplane) very well...
xxx
Then I get them a "hold on ABC VOR, radial 135º, standard turns"...
Then they do a wrong entry, and hold on the wrong side, wrong turns...
So... dont forget to review all you could be asked to do.
xxx
I want all my trainees to pass, and be outstanding.
That is my pride, as instructor or check pilot.
Especially at the debriefing, with a beer in hand...!
Again, all the best to you -
:)
Happy contrails