posted on Feb, 19 2013 @ 09:46 AM
A number of Boeing 737NG aircraft (32 in all, 17 for Alaska Airlines), all for different carriers, have had both single, and dual thrust instability
events since 2008. It typically occurs as a fluctuation between N1 (low pressure spool) and N2 (high pressure spool) speeds, during high power
operations. Usually during climb out, then it all usually returns to normal (a few flights have had one engine not return to normal).
Two events occurred as recently as August and November of 2012. During those events both engines suffered the instability, one aircraft had both
engines return to normal, and returned to the airport safely, while the other had only one engine return to normal (it returned safely as well).
Boeing says that one aircraft had both instabilities happen at the same time, while the other had one happen, and then the second at a different time.
The instability is well within the engine's ability to handle, and isn't causing damage to the engines. But has now been narrowed down to the
Honeywell hydromechanical unit (HMU) that controls fuel flow to the engines. The HMU is designed to handle a certain level of contamination in the
fuel, and it's being examined to see if there is too much contamination from one particular refinery supplying fuel.
CFM has issued a software change to the Fadec (full-authority digital engine control) that appears to have stopped the problem from occurring (there
hasn't been one since the update). The HMU has had problems since 1998, when the NG was first introduced. From uncommanded accelerations in flight,
to flame outs on the ground. A software change then fixed the problem as well.
It doesn't appear that this is a dangerous situation, as almost all the aircraft have simply throttled down for a second or two, then throttled back
up, and the problem has gone away. This will be big if it turns out to be the fuel supply though. I don't see it being fuel though, as no other
aircraft seems to have reported any problems, besides the 737. Although, there are more 737s than any other aircraft out there currently flying.
I'm still willing to bet it's the HMU or Fadec that's the problem.
Boeing and CFM International are scrutinizing fuel supplies and the Honeywell-built fuel-control system in the CFM56-7B following a series of
thrust-instability events on Boeing 737NG aircraft.
Their findings could have broader implications for the Jet A supply chain and fuel-testing regimen if contamination is behind the incidents. “We're
doing a root-cause analysis, looking at the fuel-control unit and the entire fuel supply chain,” says Boeing. “The problem might be
The instabilities, described as a fluctuation in N1 (low-pressure spool) and N2 (high-pressure spool) speeds, are occurring at high power settings,
for example in climbing to cruise altitude, after which “engine operation typically returns to normal,” says Boeing.
There have been 32 thrust-instability events since the first was reported in January 2008, with 17 of the events in the Alaska Airlines fleet,
according to Boeing. At least one incident has occurred at Southwest Airlines, based on a memo circulated to its pilots. However, two dual-engine
instabilities occurred as recently as August and November 2012. “In one case [in 2012], both engines regained normal control and performance, and
the airplane safely returned to base,” says Boeing. “In the other case, one engine's thrust did not respond properly, but the other engine fully
recovered, and the airplane returned to base without further incident.”