TFE731 Spring 2003

Engine Starting Problems - Slow and Cool Starts

— Jon Dodson

• Overview
• Electrical?
• Mechanical?
• Troubleshooting Tip
• In Closing

During the last several weeks, I have received several calls about engines experiencing starting problems. Coincidentally, all of the start symptoms were the same, so I thought this would be a good topic of discussion.

Overview

Here's our approach: determine if it is a battery/starter problem, physical binding in the engine, a fuel scheduling problem, or fuel delivery problem. First, get some information from the crew. Was it a battery start or APU/Power cart assisted start attempt? Unusual conditions? Tail pointed into a high wind situation, extreme cold, etc? Was it a Hot or Cool start? Verify the information by ground runs yourself.

Electrical?

The squawk I was hearing most was a Slow and Cool start, almost a hung start. We assume that the engine is probably not getting enough fuel, so is it an electrical scheduling problem, or is it a fuel delivery problem? If the aircraft has DEECs, perform a bite check for possible fault codes, or ECTM download, and troubleshoot per MM72-00-00.

Perform a start in manual mode, closely observe ITT limits. If the problem remains, fuel delivery is suspect (mechanical), if the problem goes away in manual mode, it's an electrical problem. Check the acceleration schedule on EEC/DEEC per MM72-00-00 Adjustment/Test. You can shorten acceleration time by: EEC - clockwise movement, DEEC - toggle the switch up. If the acceleration schedule is Ok, (per the DSC sheet in the log book) swap the EEC/DEEC and replace it if the problem follows. The DSC sheet is "Data Supplied to Customer" sheet containing all the information from the last test cell certification run. It should be in the front of your engine log.

Perform resistance checks of ITT, T2, and power lever pot per MM72-00-00 check instructions. Replace the defective LRU (line replaceable unit) or repair defective wiring if found.

Mechanical?

Now we're back to a fuel delivery problem (mechanical) if no defects have been found to this point. Check for a loose or restricted P3 line. A loose or blocked line will interfere with fuel schedule to the combustion chamber. P3 is the direct mechanical reference of N2 speed to the fuel control using air pressure as a result of N2 speed. Check the engine driven fuel pump filter for restrictions and replace as necessary, then perform a fuel pump discharge pressure check per MM72-00-00 adjust/test. If the pressure is below approximately 200 psig, and it doesn’t build rapidly, check the aircraft fuel supply system. If that checks out, then replace the engine driven fuel pump.

Troubleshooting Tip

We'll do three troubleshooting steps here at once before we go outside. We'll hook up external power capable of spooling the engine, install our pressure gauge on the engine driven pump, spool the engine in cut-off and verify the starter speed and the rapid pressure build-up of the engine driven pump. This has just told us if our batteries were suspect, if the starter was good, and if our pump is good.

Next, cap off the secondary pressure port at the flow divider and attempt a start. If the start is within limits, replace the flow divider. Do not move the throttle above idle with the secondary capped off. If you've not yet found the problem, there are still more checks you can perform, but we would probably go ahead and swap the fuel control with the sister engine at this point, and see if the problem follows.

In Closing

The Fuel Control is only doing what it is told to do, it can only increase or decrease fuel scheduling by what it is told. Many times, one's first instinct is to grab the FCU and change it. First, figure out if it is a mechanical or electrical problem, and do your best to track it down. The FCU change should be about the last step in this troubleshooting process, not the first.

For help, give us a call. We’ll either help you track it down, or we'll send one of our expert Rapid Response Team members to get you going. Spotlight is also available for troubleshooting on E-Engines.

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