top of page

#16 72-ball, ball-31-XX-XX - RTFM...

This is a bit of a boring technical post.

A field reps job is to act like a shadow manager helping the airline make the right decisions around maintaining and operating the engines they bought from us. Philippine Airlines had a total of 30 installed CF6-50 engines I think. 4 X 747s, 2 DC10s and 4 A300s. All the engines were the same basic configuration with the difference primarily being in the Main Engine Control rigging. Later in the "computer" generation reps would be responsible for hundreds of engines with multiple engine types - a degree of difficulty much higher. But this was 1985 and the world was a much bigger place.

I was assigned as a "shop rep." Just about every airline through the 90's wanted to have in-house overhaul capability and ability to enter the third party maintenance world. Giant concessions (purchase credits) were given when an airline bought engines and a lot of them negotiated things like a test cell ($20m) or shop tooling and assistance. In theory having your own shop could cut down the supply chain and one would have to purchase fewer spare engines.

At Philippine Airlines the CF6-50 would go less than 1,000 flights before needing an overhaul. For perspective the latest CF6-80C2 on the B747 could go 3-3500 flights and on a shorter range airplane like the B767 could do up to 7,000 cycles.

The engine production math is pretty simple. Divide the number of flights per year by 900, times the number of engines and you get the number of engine overhauls per year. Let's say "on average" PAL was flying 4 aircraft cycles a day - (4 X 365 X 30) / 900 = 48 engine overhauls a year. If the overhaul cycle is 60 days then 48 X 60 is the average days in process (AIP). Divide AIP by 365 and that tells you how many engines are in the shop at any given time = 8 engines. PAL had 10 spare engines so life should be good, right? No, of course not. Everyone plans for some wonderful Turn Around Time (TAT) but rarely achieve it. The logistics are daunting and PAL probably had a 90-100 day TAT, indicating a need for 12+ engines. This is AIP, there are "safety" factors applied to allow for production glitches but suffice to say PAL was always running along with zero spares with airline worst case scenarios of being grounded because there is no spare engine available.

GE at the time had a large leasing pool of engines where an airline could lease engines at exorbitant rates.

PALs capability at the time was also what we called a modular maintenance shop. They would send the major modules out for modular overhaul and disassemble and reassemble modules into engines. Except for the HPT. The HPT is the most expensive part of the engine. Highest tech and highest costs so PAL attempted to do this in house.

My job was to get into the shop and focus on how to get some spare engines together.

But first as an apprentice rep I had to earn my chops. I was first assigned to fill out "Component Change Records" and flight data as an intro to flight line operations. This was done via a Commercial Engine Service Report or CESM. There was a short form CESM on a 4 X 8 1/2 form that had like 4 copies with carbon paper copies. The long form CESM was 8 1/2 X 11 and was for major maintenance like an engine overhaul - very soon after my arrival we got carbonless CESM which was good but getting a legible print on the 5th layer was a chore. Every morning I would go to maintenance control and collect all the component change slips from a box, take them back to the office and write out in long hand all the components that were changed. It was real grunt work. Most of the changes were for sensors and ignition system parts but often there were MEC changes and other major components. Also a tremendous number of coffee makers were changed. Coffee pots had a nasty habit of catching fire a lot in the old days.

After 4 hours of writing reports I would put them all in a manila envelope and at lunch time drop them off at the post office to be sent to Cincinnati where a group of database ladies re-entered all this manual data into some IBM mainframe computer. This is where our component and engine statistics came from. In the afternoon I would hang out in the shop getting a feel for the place and building relationships with the shop guys. Mostly this was expediting new parts ordered from GE.

Every airline has a maintenance control. It is the heart of the operations. There is a flight board with all the aircraft listed and a time line of where the aircraft is and where it is going next. When a delay happens, in those days, the magnetic flight cards would be shuffled on the board among aircraft to reduce the impact of the delay that was inevitable - so an aircraft sitting on the ground for 3 hours to take the Hong Kong run might take the Brisbane run for the damaged plane and we'd hope we could fix the damaged plane before the HKG flight. We used to say that PAL INC. stood for "Plane Always Late, If Not Cancelled." If an engine change was going to happen in London then maintenance control was the place to keep apprised of everything that was happening.

The other thing in the maintenance control office was the trend charts. By mathematical calculations of the log book trend entries we could get an estimate of the engine's health. As an engine wears out the temperatures go up for a given thrust. Each take off can use a different thrust (called derate) depending on the gross weight of the aircraft, runway length and ground temperature. By using fancy math we could normalize all this to "Hot Day" conditions and plot certain parameters on graph paper starting from top to bottom. I've never been a "math guy" but I spent a lot of time manually calculating performance numbers both on wing and in the test cell.

There are a couple of engine limits. There are RPM limits for the fan and core rotors and there are temperature limits for the turbine. Fuel flow is a reference point. As the engine wears out naturally the EGT (Exhaust Gas Temperature) goes up. The redline is about 960*C and you are allowed 10 excursions slightly above this. Due to thermodynamics you can get a bloom in temperature at TO because the metals grow at different rates and clearances can change. For every 0.010" of clearance in a turbine the EGT can change by about 10DegC.

Of course as EGT goes up, fuel flow goes up. The other thing is the VSV and VBV systems. The blades and vanes if in a fixed position would give a fixed amount of pressure/compression per revolution. In order to make the engine operate over a broad range of speeds the compressor is designed to produce enough air to make TO power - The core engine being a fixed diameter pipe that can only pass so much air. At low speeds the compressor makes too much air and an engine stall can result. Pratt designed their engines to dump compressor air overboard. One of GE's brilliant founders designed the Variable Stator Vane system. Like a prop plane the pitch of the vanes is variable. At low speed the vanes are closed taking a smaller bite of air and at high speeds they open up to a coarser pitch. This is controlled by the Main Engine Control or MEC. The low pressure compressor attached to the fan also produces too much air and in the case of the fan GE also had a Variable Bypass Valve system which was a series of doors on the fan that opened up and dumped air from the core flow into the bypass flow.

The pilot needed to be oblivious to all this so GE based thrust on fan speed. If the fan was turning at X speed then take off power was assured. But the fan and the core were not coupled. There was an aerodynamic relationship. The more fan speed you wanted the more core speed, or technically core flow you needed.

As the engine wore out you needed more core engine speed to achieve a given fan speed due to sealing losses, so N2 (shorthand for core rotor) speed went up and the VSVs operated more closed allowing the core to spin faster - kinda like 1st gear vs. 4th gear in a car. But there was an N2 speed limit and you could get core speed limited. The solution was to open the VSV schedule to get more air at a lower rpm - but this is like starting you car in 4th gear and a stall can happen so it was a limited tool. Generally to get more air you closed the VSVs a little and let the RPM run higher. Fortunately unless the compressor was damaged the engine reached EGT limits before N2 limits.

All of this magic happened in the MEC. A Rube Goldberg hydro-mechanical device of some very clever stuff. The hydro was jet fuel. The mechanical devices were a couple of governors and a magical 3 dimensional cam that set the locations of the valving and so on. It took inputs from temperature senders and pressure senders to adjust the fuel flow based on what the engine temps and pressures were doing. There were idle speed adjustments, "part power" adjustments and Max Continuous adjustments as well as a "rig plate." The rig plate basically adjusted for different variants of the engine and/or the components in the engine. As the engine evolved there were a bunch of rig plates. The procedure was to put a pin in the throttle pulley in the cockpit and a pin in the pulley at the pylon to ensure the throttle cable was rigged right and then using a screwdriver adjust the rigging so that a target line was lined up against a certain point on the rig plate.

So part of my job was to get the log book transcripts and update the trend charts. Later GE developed some circular slide rules that made the calculations a lot quicker.

In the afternoon we would look at each engine trend. Rising EGT and fuel flow coupled with declining N2 would indicate an engine wearing out. A step change in parameters could indicate something in the engine broke and it is time to have a look inside with a borescope. It was very common to ingest debris in the compressor or birds causing damage to the compressor, loss of efficiency and high EGT. It was also common in high time engines for the turbine to start burning up and cracking.

We were expected to know a lot and make decisions on our own but we were not alone. At headquarters every customer was assigned an Airline Technical Programs Manager (ATPM) and a Customer Support Manager (CSM) the CSM was the business guy handling all nature of claims, issues and business problems. The ATPM was the technical lifeline. When you didn't know the answer you sent a message to the ATPM and hopefully the next day he sent an answer back.

This wasn't the days of text messages and computers. The fax machine hadn't even been invented yet. We typed messages into a telex/teletype machine that created a ticker tape. Then we loaded that into the ticker reader, dialed a number, turned on the print function and hit send when the connection went through. The printed record would go in a file and be matched up with an answer or answers and kept for reference.

The telex machine and Philippine phone system was a constant source of trouble. It is well known that Mike Anderson - John's predecessor at PAL walked out of our office on the mezzanine floor of the PAL hanger carrying the telex machine to the top of the stairs and chucking it overboard. The reps in those days were the epitome of "white privilege" and we were rarely questioned about our craziness. As my buddy Jeff would later say we were like fire gods - knowing the unknown secrets of the universe.

But we had Gods we looked up to. Johnny Dodge was our ATPM. He was the male equivalent of Millie Bacon. He was about 900 years old and knew everything. He was a heavy smoker and drinker, was about 5' 8", skinny as hell with perfectly white hair and a mouth full of dentures. No ATPM suffered fools well and they were quick to point out guys that couldn't cut it. If an ATPM didn't have the answer he had access to the entire engineering department of specialists to dig up answers.

Johnny loved to come to Manila. He would wear a Barong Tagalog and as soon as he arrived in the office he'd want to go tot he airplanes and get involved. Airplanes would be in the hanger for a scheduled check and a BSI - Johnny would whip off his Barong leaving only his wife beater and stick his eye in a turbine blade borescope. "Oh, shit. That bitch is burnt to fuck. It's only got a half a fucking cycle left!" - a half a cycle indicating it would take off and blow up in flight. He would then go into maintenance control, find the trend where EGT was going up and N2 was going down, drawing a diverging set of lines on the graph. "Look at that fucking whore with her legs spread apart. That engine is fucked!"

The manuals are arranged in a chapter verse system agreed upon by the Airline Transport Agency or ATA. You can locate everything by it's ATA code. For example the compressor rotor was Chapter 31. In general the on-wing chapters of simple stuff to keep the engine running was in Chapter 72(engine)-00(engine level)-31(compressor chapter) and sub chapters for the various components like blades and disks and bolts and air screens (remember those). For the in depth or shop stuff it would be 72-31-XX getting nitty gritty on repairs to the components.

The stupidest one could feel is to send Johnny a telex that was thoughtful, intelligent and clear and wait patiently the 12 hours it took to sleep in Manila and answer a question in Cincinnati only to receive back, "72-00-31-XX-YY. RTFM!" Which basically said, "Here's the chapter, paragraph and verse where the answer is. Read the Fucking Manual"

The ATPM community is also close nit as they all sat in cubes of 4 guys. I had a protoge called Pat Pogue. He was a real piece of work and way more confident in himself than he should have been. For context the engine cowling encompassed the Fan Reverser - the cowl would translate back, raising blocker doors in the flow path and through turning vanes direct the air forward. The reverser was like a big balloon in two halves - they rolled back separately but when pressurized they came together at the bottom and "bump stops" on each half mated together to take the load. Imagine the bumpers on a train. very similar looking. The reverser system was another Rube Goldberg piece of work, very complex. But one thing was pretty simple. The manual said when rigging the bump stops the static gap should be about the size of a pea.

Well I was on a trip to Cincinnati and Pat was home alone. I think PAL was fucking with him because they definitely knew how to rig reversers. Anyway they asked what size pea and Pat wanting to know the airspeed velocity of a swallow and how many coconuts one could carry drafted a message for Johnny. However he took the opportunity to explain how terrible the manual was and confusing and so on.

So when Johnny gets the telex, he tosses it to Clarence Hitchcock. Clarence was the reverser expert and had responsibility for writing the manual in terms of rigging and maintenance.

I am sitting in the CSMs office and the phone rings. I hear with no speaker phone, "Who is this Pat fucking Pogue and who does he think he is and should I call Schneider to find out what kind of dickheads we are hiring into field service or what." Clarence was hilarious and a real expert. KT, the CSM, said, "Glad you called. Dan Deutsch is here in my office and he is on the way to you." Thanks KT. I go down and the bullpen is laughing and devising a response to Pat. Once they realize he is a newbie and not connected to anyone politically the fun begins.

I would never do Clarence's response justice but it probably had to do with parentage, having any kids who lived and whether the parents might have been cousins. Clarence even suggested that the range of pea size when rigging a 9 foot diameter reverser might not be so fucking critical and if one had half a brain one could figure it out."

It was a good enough response that when I got back Pat asked me if he was gonna get fired. I told him probably but not this week and told him how I laid on my sword for him and he owed me big time. Fucking with people was a fun part of the job. Pat never did learn his lessons though. He seemed to like "getting hit on the head lessons" he finally jumped ship to work for Aloha Airlines and was just as stupid as a customer as he was as a rep;

So when in doubt - RTFM MF"

Here is a schematic for the CF6-50 MEC. It was a complicated bit of kit but there were only a few adjustments that could be made on it. Computerization with later engines made rigging knowledge obsolete. Two of my old time buddies, Tim Clark and Tom Kirwan are based at FedEx. Cargo companies take the oldest planes used and fly them forever. FedEx finally got rid of their last CF6-6 and still operate the CF6-50. I think Tim Clark is the only guy left in the world that can rig a CF6-50 MEC and/or reverser.

There are ATA chapters for everything on the airplane. A lot of airlines and airplane makers view the engine guys as simply the largest "Line Replaceable Unit" (LRU) on the airplane. Of course while technically true the engine is probably the only component that stays on wing and gets all kinds of maintenance done to extend its life. Not like a brake or an actuator or an autopilot computer and so on.

Begrudgingly the aircraft makers know that the engine can be the key factor as whether an airplane is successful or not.

13 views2 comments

Recent Posts

See All


Fabulous stuff and i even think that I understood some of it too. There's a lot of acronyms to remember but I think that's true in any industry, back when I was working in Social Security we had to keep track of LAWPs (Liquid Assets Waiting Periods) and PECs )Pre-Elegibility Claims) so it's all the same.

I still think that it's amazing that jet engines wear out - I mean I know it must be true but in my old world of electronics nothing ever wore out it just stopped working, sometimes in spectacular fashion. One job I had was building circuit breaker re-closing units, after the build I had to manually test them by charging them up to som…

Dan Deutsch
Dan Deutsch
Aug 11, 2022
Replying to

Yeah - Acronyms are a huge part of the mystery. A good rep can remember a ton of acronyms. A better rep knows what they mean - LOL...

You hit the nail on the head in regards to maintenance. We used to say, "This is the airline business, not the airline hobby." Everything needs to be done with safety first, then cost second.

Dollars per engine flight hour is basically the standard metric. It isn't good to spend 4 million bucks on an engine that lasts 2,000 flights when you can build an engine for 5 million bucks that lasts 3,000 flights.

The problem is that the airline finance guys are quarter point driven - Convincing them to spend $48m…

bottom of page