Helicopter Maintenance Blog*
Posted 4 years 213 days ago ago by Scott Skola
Looks like 2017 is the year of legacy retirements. Last month the Indian Army elected to retire their fleet of HAL built Lamas and Alouette IIIs. Now it hits closer to home.
The last UH-1 (S/N: 74-22478) made its final official flight as an U.S. Army aircraft at the end of 2016. This marks the end of a 42 year history with the Army.
I wonder who or what is next?
And away we go…
TIPS AND TRICKS:
Airbus (Sud Aviation)
This tip originated on an SA316B Alouette III. The Alouette III is the master aircraft on the Type Certificate H1IN, of which the Lama is a version. The tip is applicable to the Lama and any other helicopter or aircraft with a similar fuel system setup.
The Alouette III and Lama have a boost pump at the bottom of the fuel tank to provide positive fuel pressure and delivery to the engine-driven fuel pump. Purging of the fuel system and visual inspection of the fuel for contaminants, a daily check, is done with the boost pump on and by depressing the purge valve on the fuel filter, and capturing some of the sample.
The pump is turned ON by a switch on the instrument panel, however, to mitigate a situation in which the boost pump is not turned on, it is on automatically and remains on when the automatic engine start process is begun by selecting “START”.
If the boost pump fails in flight you must descend to below 5000 meters MSL to continue flight per the flight manual. If the pump fails on the ground, you can still start the Artouste 3B1 engine and operate under 5000 meters MSL. However, if air gets in the fuel, you will not be able to purge the system nor start the engine.
The logic behind the allowance of an operating limitation with the boost pump inoperative is based on the physics that the fuel in the lines will remain reliably in a solid column up to a point where the ambient air pressure cannot exert enough force above the fuel in the tank to maintain the column of fuel going to the engine-driven fuel pump (which is also providing some suction).
So if the boost pump becomes inoperative, you must forego the operation of the daily purging of the fuel system or you’ll introduce air into the system and not be able to purge it, and not be able to start the engine.
This is exactly what happened to an Alouette III that was operating at sea level in the Middle East. The boost pump became inoperative in flight (illuminating the fuel pressure light) and the pilot knew he could continue operations under 5000 meters MSL, no problem. He reported the inoperative boost pump to the mechanic at the next landing for fuel and flying continued the rest of that day.
The next morning the pilot arrived at the aircraft and sumped fuel off the bottom of the tank. He then promptly by habit turned on the battery, turned ON the boost pump and when he depressed the plunger on the fuel filter purge valve he knew he’d made a mistake when it just made a gurgling sound.
He told the mechanic what he’d done and assumed they’d be off that day, and likely for days until a replacement boost pump arrived, which the mechanic had ordered the night prior, but was days away from receipt.
After assessing the situation, the mechanic knew all he needed to do was pressurize the fuel tank enough to purge the air out.
He brought a tank of compressed air to the aircraft and applied air pressure to the fuel tank vent, sealing the connection of the air gun and vent tube with a rag. This increased the air pressure in the space above the fuel in the tank and enabled the pilot to purge the fuel system and subsequently start the engine and continue flight operations.
The mechanic avoided some days of down time by being analytical and creative. He also saved the pilot some embarrassment and blame for some days of lost revenue.
[Submitted by Lama-Nator]
Some more Robinson stuff. Blank PDF maintenance forms:
SUBMITTING MAINTENANCE TIPS/TRICKS/QUESTIONS/INFO:
Have an old tip or trick you’d like to share with your fellow mechanics? Or maybe a question that you can’t seem to find an answer to? Or just some info to pass on? Send an email to: [email protected]
About the author: After 32 years maintaining helicopters in various capacities, Scott concluded a full time career with a major operator in 2014. When not pursuing future writing projects, he can still be seen around the flight line tinkering on aircraft for beer money.
*To keep the hounds at bay, the information contained in this blog is for discussion purposes only.