Jumpseat: It’s All About the APU
For more than 50 years of jet airliner operation, the APU has been an integral part of airplane independence. The APU is a small jet engine located within the structure of the fuselage. With today’s airliners, the unit is operable both on the ground and in flight. In flight, the APU provides both a backup source of electric power and a limited amount of air pressure at lower altitudes — usually below 20,000 feet.
On the ground, the APU is capable of being the sole source of electricity and the sole source of air pressure. Air pressure from the APU is the standard method utilized in starting the engines. When air is not available, starting becomes problematic. On this particular trip, an inoperative APU became more problematic than my copilot and I could have ever imagined.
As was my typical schedule, London was the destination with an 18:15 departure from JFK. The trip is our airline’s only evening departure without a relief pilot. If the stars align, the 6:30 en route time is a nonevent for a two-man crew. Unfortunately, the stars misaligned the moment I attempted to retrieve our flight plan data. The dispatcher had locked us out of the flight plan. Why?
On most occasions, the only contact with dispatch is our electronic signature. The signature indicates that we agree with his assessment of the flight. Verbal communication is rare. But our trip had a problem. The dispatcher wanted to ensure he got my attention. I had to actually use real verbal communication via a phone line.
The problem involved two mechanical issues in addition to weather issues — more on the weather later. One of the mechanical problems was an inoperative scavenge pump for the center tank. The scavenge pump transfers remaining fuel from the center tank to the two main wing tanks. If the pump is nonfunctional, an imbalance situation might occur. Our fuel load to London rarely, if ever, requires fuel in the center tank — problem solved.
As mentioned, the other discrepancy was the inoperative APU. If it’s not available, then the MEL requires the flight to remain within 180 minutes of a suitable airport at all times. Again, not a problem across the North Atlantic.
All that being said, our ramp service personnel seem to find it difficult dealing with a 777 and no APU. It’s a rare occurrence. And on an airplane of that size, it becomes a major inconvenience. The concept that an external source of air for engine start (start cart) and a source of air for cabin climate (preconditioned air) are two different items seems to be a bone of contention. The fact that air from the start cart pressurizes the same plumbing for both start and cabin climate often goes beyond comprehension.
In addition, the need to maintain our connection with ground electric power throughout an inoperable APU start is another source of confusion. The ground crew often asks me to remove electrical power just prior to start, a normal request for a normal situation. But, unfortunately, without an APU, the airplane will become very dark.
Due to my discussion with the dispatcher, we arrived at the gate later than usual. After distributing our bags to the appropriate places, my copilot exited the cockpit to complete the walk-around inspection. During his absence, a mechanic appeared with our maintenance logbook. I innocently asked if any of the MEL items were fixed. His response was a snicker.
Upon the copilot’s return, we were greeted with the rattling, high-pitched scream of the start cart. The noise was soon accompanied by the wonderful smell of diesel exhaust. It was 20 minutes prior to departure, and we were getting gassed. And to add insult to injury, the outside temperature for this particular December night was 56 degrees. Without conditioned air from the APU, body heat from 184 passengers climbed the cabin to above 80 degrees.
Why not use the start cart to supply air to our air conditioning packs? The cart was running but not activated in a mode to supply pressure. The ground crew couldn’t be reached. We discovered later they were scurrying about trying to sort and load bags from a previous ground crew that left them a disorganized mess. Progress was slow.
When communication was finally established with the ground crew, the circus began. Over the interphone, I indicated that we were getting asphyxiated. The prevailing wind and the position of the start cart just below the forward entry door underneath the jet bridge was a safe place for ground personnel but not helpful for breathing purposes. Explaining this circumstance and finding a solution was a painful process that took us even further past departure time.