Brakes on Boeing 737 Prototype Airplane
Bob Bogash
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What does a Yugoslav 707 and a NASA 737 have in common?  Maybe nothing; maybe something. Maybe several things.

The brakes on the Boeing 737 Prototype Airplane - the airplane in the Museum of Flight - are connected backwards.  Yes, it's True.  Somewhere in the airplane the plumbing has been reversed. 

I don't know if they came out of the factory this way and have been that way for its whole life; or were crossed at some point during its service life. 

NASA 515 (PA099) at Moses Lake - MWH

Here's the story:

It's quite difficult to check this out, which is why it may have lain undiscovered, and I only came on it by accident and some degree of ornery perseverance.

On August 13, 1972, a JAT (Yugoslovenski Aerotransport) Boeing 707 suffered an accident at JFK Airport in New York. The airplane was taking off from Runway 13R, when the Capt. elected to abort the take-off at high speed (3 seconds after V1 which was 150 kts). The co-pilot's sliding window had popped open.

Note: This exact same event happened to me during a high speed - over 100 kts - test run on Runway 32R at Moses Lake in NASA 515, the 737 Prototype. I immediately thought of the JAT accident; Brien Wygle - pilot of that airplane's First Flight in April 1967 - was acting as my co-pilot at the time it happened to me. With the engines running at Take-off power, and the wind noise at high speed, I can confirm the event can best be described as being "Explosive!!!"

The JAT 707 (a -321 model, former PAA), overran the end of the very long runway (14,572 feet), struck a blast fence, caught fire and incurred significant damage to the left wing and engines.  It appeared to be a write-off, but was eventually repaired and returned to service.

I saw the accident airplane - YU-AGA in Honolulu in 1974 - when it was performing a Round-the-World charter flight. I'm trying to dig out photos I have of it at that time.

There were 16 minor injuries to the 170 occupants of the aircraft. The take-off was being performed at Max Take-off weight of 312,000 lbs. The crew performed the RTO properly, using speed brakes, 100% reverse thrust, and maximum braking. There were skid marks for over 6300 ft along the runway.

Performance data indicated the airplane should have been able to stop with 2500-3000 ft of runway remaining.

In one of literature's greatest detective stories - The Hound of the Baskervilles - detective Sherlock Holmes was able to solve the mystery because of a vital clue - a dog's barking. Actually, the clue was that this particularly ferocious dog DID NOT bark - indicating his familiarity with the villain.

Beware of Dogs that Don't Bark
Brakes that Don't Brake

Air carrier airplanes are maintained using several systems. Some units are Flight Time, Cycle, or Calendar Time limited, after which they are removed and cycled through repair shops, or retired. Most units are O.C. - On Condition. They are removed for cause due to failure, or reported discrepancy.

Some units are clearly O.C. - and these would include Tires and Brakes. These components wear out steadily during use, and their condition is checked by the flight crew (and maintenance) during walk-arounds before every flight. Tires are routinely replaced when observed worn, so that the tires on a given airplane all exhibit varying degrees of wear. The brakes likewise wear down during use. They can be checked readily by observing protruding pins, known as wear indicators. Like the tires, the various brakes (8 in the case of the 707), can all show varying degrees of brake pad wear remaining because they get changed at different times, when required. During the pre-flight walk-around, flight and ground crews merely observe that the wear indicator pins show adequate wear remaining.

Inspection by the NTSB after the accident revealed that two of the tires and especially their associated brakes were not worn. The brakes, in fact, had not been used for some prolonged period of time (maybe forever?) In normal service, operation of the airplane using only the six functioning brakes would have been easily possible, with no indication to the pilots that two of the brakes were not functioning. The cause was determined to be a malfunctioning anti-skid relay that rendered those brakes inoperative above 20 kts. This was particularly insidious since below 20 kts, the brakes operated normally, and a ground maintenance check would not have detected a problem. Because of the regular replacement of brakes and tires, no tracking system was in place (and may not be even today) that would have flagged that these two brakes had never been changed. Pan Am performed all the maintenance on this airplane under contract with JAT.

In the case of this RTO, although maximum braking was applied, the energy required to safely stop the airplane exceeded the energy capability of the available brakes. In fact, the six remaining brakes were all destroyed during the RTO, with brake part debris strewn back along the stopping path. The destruction of the good brakes further degraded the stopping capability of the airplane. In the end, the airplane could not have been stopped on the runway, even though (at the time) it was the longest commercial airport runway in the world!

On the 737, (at least the -100/-200), the "A" system powers  the Inboard brakes, and the "B" system powers the Outboard brakes.  The "A" system is powered by two EDP's - Engine Driven Pumps - one on each engine.  The "B" system is powered by two Electrically Driven Pumps - both in the MLG wheel well.  (This has since been changed on later models to one Engine driven/one Electric pump on each system.)

Well, in normal service, if you want to pressurize the B hyd sys, you turn on the B-pump switches on the overhead.  If you want to pressurize the A sys, you set the Parking Brake and open the Ground Interconnect Valve.  But - when you do that, you pressurize both A and B systems.  There is no way to pressurize just the A sys alone without running the engines.  The same applies if you use an external hydraulic power source (a mule) through the Ground Power Module.

While I was maintaining the airplane at Moses Lake, we did several wheel and brake changes.

Getting ready to do the wheel and brake change with my pal and great volunteer  Steve Huemoeller

Nice shiny new brake in Nbr 2 position - Man - they are heavy!

Well, I'm not sure why I'm smiling - guess I love my job!
The first "new" brake had "issues" and we had to change it again.
Fortunately, we had several spares.

Afterwards, I wanted to do an Operational and Leak check, so I powered the B sys pumps and opened the Ground Interconnect.  Since this was an Inboard brake, it ran off the "A" system, and I had to pressurize it (the A system) by opening the Ground Interconnect.  Then I cycled the brakes.  While looking at the brakes on the ground, when brakes were applied, all four brakes applied, and when released, they all released.  No leaks.  All was good.  Or was it?  This is what would/could have happened during its entire service life.  We'll never know.   But, in this case, I decided to check further.  Damn engineer!

For whatever reason, I closed the Ground Interconnect, depressurizing the "A" system, and repeated the tests.  I was surprised when I did that --  the Inboard brakes operated but not the Outboard brakes.  I was confused - B system operates the Outboard brakes and they were not operating.  It seemed to be backwards.  There was only one way to confirm this.

We taxied out and did several brake applications to "break in the new brakes."  Next, we did a high speed run down Runway 32R to the north and then parked on the Echo taxiway holding pad with the engines running.  I turned off the B pumps, made sure the Ground Interconnect was closed, exited the airplane and re-did the check with the engines running and just the A sys pressurized.  Bingo!  I found that applying the brakes activated the  OUTBD brakes - it was supposed to be the INBD brakes.  They were indeed reversed!

Back at the ramp, I dragged out the manuals and confirmed that my memory was not faulty.  Yes!  It was true.  The brake hydraulic systems were indeed reversed!

I tried to analyze what the effects of this might be.  If any.  All the brakes were receiving hydraulic power - just not from the intended system.  As long as everything was semi-normal, the pilots would never know the difference.  However, a  problem might arise on a slippery runway when the anti-skid would be applying and releasing brake pressure to individual wheels.  In that case, presumably, brake pressure might by cycling the brakes on the WRONG WHEEL. I emphasize "MIGHT."

This airplane was used extensively for slippery runway testing, so it's unknown how this all played out during the tests, or if perhaps, the brake lines were not reversed, or got reversed after that time.  Or was it that way from Day One?  Also - NASA 515 has a Third hydraulic system - altho ostensibly just for the flight controls.

  You had to be pretty persnickety to uncover this little gremlin.  I plead GUILTY, your Honor!

In the factory, or at a major maintenance base, they can use external hyd press hookups, but that system apparently has the same shortcomings as using just the B pumps in the field. In service, operators normally use the B sys pumps to do that job and so are unable to do a Sys A ONLY check. The ONLY way to only pressurize Sys A on the airplane is to run the engines, with the Ground Interconnect Closed, which is what I did.

I wasn't interested in tracking down where the tubes had been crossed, so left it that way and that's the way she is right now.  And might have been that way her whole life.  In fact, other -100/-200 airplanes might be like that too - maybe ALL of them!

I have spent quite a few hours studying the schematics trying to figure 1) where the line switch might be located, and 2) what the impact would be on the braking and anti-skid functions.  It seems, at first blush, that manual braking would be normal, but it gets hazier when you consider both anti-skid and auto-brakes.  The plumbing through the Metering Valves and the Anti-skid Valves is a bit more than meets the eye, with, amongst other things, the hyd fluid returns potentially going back to the wrong system reservoir. If any grey-beard readers out there can shed light on this, I'd welcome their inputs.  Study the schematic.

So, at the beginning, I asked what do a Yugoslav 707 and NASA's 737 have in common.  Well, they both have experienced the co-pilot's sliding windshield popping open at high speed.  And, they also both have/had hidden and unknown discrepancies in their brake systems.

Copyright 2021 Robert Bogash.  All Rights Reserved