Aviation Checklist Confusion: A Pitfall ATR Pilots Must Avoid!

A week ago, I published a video about Emerald Airlines flight 3671 (see link below.) During arrival to Belfast City airport, two important electric busses lost power. This resulted the loss of several systems needed to fly the approach and land. Thanks to a sound understanding of the aircraft, the crew managed to solve the problem and landed safely.

When I was working on that video, I came across the “Standby Busses and Battery Power Only Lost Equipment Lists” in the QRH. The more you look at it, the more confused you might became. To understand the logic of the lists, we must understand the basics of the electric system.

Index
00:56 - A peculiar (check)-list
01:55 - ATR electric systems (a quick introduction)
05:26 - Losing the DC generators
06:53 - The “Standby Busses and Battery Power Only Lost Equipment Lists”
09:39 - Bonus: Landing at Koh Samui, Thailand

Links
Emerald Airlines Flight 3671:    • ATR 72 Emergency Landing: Uncover Essentia...  
ATR electric systems:    • ATR electric systems - an introduction  
The TRU:    • Why are ATR -600 pilots using the TRU? | A...  

There are 2 checklists referring to “The standby busses and battery only lost equipment lists”: "DC generator 1+2 fault," and "DC standby bus failure." When both generators fail, the main battery will power the ESS bus, and both STBY busses. And the emergency battery will power the EMER bus. If the generators cannot be reset, we activate the TRU.

If the TRU doesn’t work, or if the aircraft does not have a TRU, the maximum flight time in IMC is limited to 30 minutes. This is the time it takes for the main battery to drain out. We also set the battery switch to override. This is a precaution in case the STBY busses didn’t switch to the main battery as they should.

When the voltage to the DC STBY bus is below 19.5 V, we get an UNDV alert. This means the main battery is almost empty, and we will soon lose the DC ESS bus and both STBY busses. Then, we are left with the emergency battery powering the EMER bus. All we have left, are enough systems to let you fly the aircraft, and talk with ATC, who can see you on the radar. But you cannot fly an instrument approach and land. To do that, you need to read the checklist for "STBY bus UNDV" which tells you to select STBY BUS OVRD.

Here is one important thing to remember: When those busses lose power, several indications and alerts will also disappear. Therefore, it is important to follow the checklists carefully.

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