Thursday, November 8, 2012

Afterburners and end users

I was recently reminded of one of those great stories that also has a useful lesson buried it. But first I just want to talk a little about the relationship between the user and the maker.

In many cases the user and the maker are one in the same person. Similar to the squareness comparator gage project in a previous article on this blog. It is not for a customer or client, and it is not intended to solve any specific problem other than the problem of my need to built things. Part of what I do in this line of work is listen to people tell me about their technical problems some they would like solved and have some fun in the process. This problem solving process between the user and the maker has some built in potential pitfalls.   

Sometimes I find it very easy to get seduced by a nifty technical problem and expend more time and energy than the user had in mind. In fact if the users had some idea how much time you might spend on their problem they probably would never tell you about it in the first place. We might built a pilot plant when all they had in mind were a couple of buckets with a hose between them. It is certainly easier to avoid the business problems if your not on an owners clock with the solution. This is sometimes referred to overkill or over engineering and is deadly to the user, maker business relationship. If you are a one person operation and it matters little if you spent one hour or a hundred hours on the problem because its transparent to the user who uses the cost as a yardstick. The only real damage is you end up working pretty cheap sometimes. 

It can be especially difficult for people like me that really enjoy this work and do it for the fun of problem solving and working with my hands. A real interesting problem has the same effect on me as a time machine. I look up and hours have flown by that only seem like minutes. The reason for the time dilation effect is that I use my toolmaker friend Charlies rule, whatever I spend my personal shop time on has the requirement that it has to be stimulating. As designers and makers we can be easily lulled into thinking we fully understand the problems users bring to us. After all this is our job, to figure things out that other people are having trouble with. I am guilty of focusing my attention on what my experience tells me the problem is. After many years I have learned to trust these instincts and rely on my own judgement even though I'm many times wrong after a deeper look. The story I mentioned at the beginning is one that illustrates this well.

A workmate of mine told me this story about a situation that happened when he was in the air force. His job in the air force was aircraft maintenance and electronics. In fact he was a crew leader assigned to maintain a squadron of F4 Phantom fighter bombers. I never got where this all took place but I think it was an air base in Germany somewhere. The crews duties were to keep an airwing of these complicated machines repaired and maintained during the cold war. They had the training, tools and the resources to strip one of these aircraft down to the airframe if necessary. As part of their job they regularly changed engines and did full engine rebuilds. The flight mechanics out in the field like my friend Jim probably knew these machines better than McDonnell Douglas did. When you spend this much time with a particular machine you get to know it like a spouse or child, learning all its moods and behaviors. One of their duties was to respond to user (pilot) reports of maintenance issues or malfunctions.

One day a repair ticket comes in on one of the squadron F4's. The pilot complained that the throttle control was malfunctioning and he could not get his afterburners to engage. Part of the process was to talk to the pilot and get as much information as possible before doing the repair. As I understand how the throttle works on the F4 based on my friends description there are two independent slides that are advanced to increase thrust in both engines. Near the maximum throttle position there is a little micro switch that closes to send a signal to the engine control to ignite the afterburners. After discussing the problem with the pilot he was pretty sure this switch was the problem based on his experience.

They opened up the left console and performed the recommended check on the switch. Small surprise, the switch passed with flying colors. I guess there were several other things that got checked during the repair but all their checking didn't pinpoint any problems. Everything got put back together and the crew signed off on the repair as good. They returned the plane back to flight status and gave it back to the pilot.
The next day they got an irritated call from the pilot asking why they didn't fix the problem. He still could not engage the afterburners even after their "repair" job. My buddy Jim explained that they had thoroughly checked all the afterburner systems and found no problems. He described all the tests that had been performed and whatever minor adjustments had been made. They would have to take another look and see what was going on.

After they got their hands on the plane again the crew performed the same checks they did the first time. Just as before they found everything functioning to specifications. Reaching for ideas they decided that the problem might be related to vibration or heat when the engine was running during flight. It was decided that they would do a chain down test with the engines running. A full power engine test on the ground is a big deal. You don't just throw a rope around the trailer hitch and loop it around the oak tree next to the garage. The procedure is complicated and dangerous to the ground crew who have to get close to the machine when its running at full power.

Each one of the Phantoms General Electric engines produces 18,000 pounds of thrust in afterburner. The procedure is to chain it to the flight line in a special area using heavy duty chains arranged in a particular configuration. I'm taking a guess here that the chains didn't come from the McMaster Carr catalog. After getting the proper clearance the crew went through the lengthy procedure dragging the gear out to test the planes engines on the ground.
An F4 in full afterburner in a test stand somewhere gives you an idea of what these guys went through.
So they are set to go with the test. They run the engines up to full power slowly to make sure the rigging gear is good and there are no surprises. With a final click the throttles were pushed past the little micro switch and low and behold the afterburners kick in boosting the load on the chains to nearly 40,000 pounds. They try it several times since they spent all this time on the ground engine test without a hitch. After shutting it all down they pronounce the engines, plane and systems are all functioning as designed. As a precaution they even replaced the switch to put any doubt to bed. They returned the plane back to the flight line and tell the pilot everything is functioning properly and has been tested thoroughly.  The pilot hearing this seems satisfied especially after hearing the test that his plane had been put through and the switch replacement. He told the my friend that he had a flight the later in the week and would let him know how it went.

Around this time my friend is pretty sure of himself that the switch is fine and functioning and his assessment and diagnosis were correct. What is bothering him slightly is that the problem might be an intermittent problem that they just couldn't reproduce. Those thoughts went out of his head as soon as the next plane came in for work. He was satisfied that he had understood the problem and done everything technically correct to fix it.

I don't know if you have ever met a fighter pilot that's a high ranking officer to boot. Lets just say they didn't get where they are from being meek and conservative. I think they actually take training on how to ream somebody out until all that's left is a smoking pile that used to be the focus of their attention. I'm sure it wasn't a gentle summer breeze blowing in Jim's direction. The first clue they got that the problem wasn't resolved was a loud angry phone call from the pilot (major so and so) barking at them for not fixing a simple problem. After going up one side of my friend Jim and down the other the pilot told him to be front and center to go for a test drive with him the next day. Now I would really like to go for a ride in one of these but even my enthusiasm is damped at the thought of an expert pilot that is pissed off at me giving me a ride in a high performance machine.

Jim shows up the next day all kitted out for his test drive in the jet with the mysterious problem. The pilot is in a hurry to get on with the demonstration so they load up and head out to the runway. They taxi out to the runway and get in line to take off. The pilot asks the tower for clearance to do a maximum rate climb out after takeoff which they granted. The little glurping sound is Jim's guts tightening up. During all this he is pretty sure the afterburner is going to kick in since he just tested it a few days before. The technician geek in him is watching the pilot from the backseat of the fighter for any clues to the problem. Finally its their turn to take off.

The pilot puts the brakes on and throttles up to take off power but not afterburner. Finally they are off accelerating down the runway. The plane gets its wheels off the ground and the wheels up safely when the pilot announces "You ready?" Jim has been ready from the night before so he braces himself for that additional 12,000 pounds of thrust from the afterburners he positive are about to be lit. He watches the pilot grab both throttles and slam them to the firewall in about a millisecond. Pfffffffffff. No afterburner. In an instant Jim realizes what the problem is, but right now he has to listen to another reaming session this time over the headset from the pilot about wasting his time and not believing him that the problem even existed. I don't know how long the ride lasted but after that it couldn't be short enough for me.

When they performed the tests on the ground they advanced the throttles like any good technician would, slowly and smoothly. In his shoes I would have done the same thing. The pilot on the other hand who's job it is to make instantaneous life and death decisions just wanted his power to happen NOW not thinking about treating the equipment with like a newborn baby.

I'm not sure what the exact problem was with the switch but it was related to the length of time the signal from the switch could be read. The ground crew solved the problem by lengthening the tab that contacted the switch which increased the closed time of the switch. This added a few more milliseconds for the logic controller to read the signal. I may not have all the details of the story perfectly correct but the moral is the same. Whats the moral of this story? Moral number one, the user and the maker have different needs.

Second, don't let yourself get seduced by a technical problem. Mother nature never cheats or lies to us, but she will let you make a fool of yourself because you are the easiest to fool.



Cool description of a similar test.

1 comment:

  1. Not even McDonnell Douglas can engineer out recurring problems between the seat and the steering wheel!

    ReplyDelete