Interesting incident

[EMIT]

Quote from Aviation Herald.



Incident: Airbridge Cargo B748 near Hong Kong on Jul 31st 2013, both left hand engines surged at same time, one right hand engine damaged too

By Simon Hradecky, created Tuesday, Aug 6th 2013 17:24Z, last updated Wednesday, Aug 21st 2013 16:54Z

An Airbridge Cargo Boeing 747-800, registration VQ-BGZ performing freight flight RU-349 from Moscow Sheremetyevo (Russia) to Hong Kong (China), was enroute at FL410 about 80 minutes prior to estimated landing when both left hand engines (GEnx) surged simultaneously, engine #2 (inboard) temporarily shut down and auto-started again. The aircraft continued to Hong Kong for a safe landing on all 4 engines about 80 minutes later.

Rosaviatsia reported on Aug 6th 2013 that the aircraft entered a zone of dramatic temperature fluctuations, the temperature changing between -53 and -33 degrees C, when the engines surged, #2 spontaneously shut down and auto-started again without crew intervention. Both engines received damage to high pressure compressor blades.

In a safety message to operators released on Aug 20th 2013 Rosaviatsia rated the occurrence a "very serious incident" reporting that engines 1,2 and 4 were affected. Ice accumulated on unheated parts of the high pressure compressor in unpredictable and poorly understood meteorological conditions leading to the disruption of air flow inside the engine causing surges of the left hand engines. The flight was continued to destination, a post flight examination showed "unacceptable damage" to the high pressure compressor blades of engines 1,2 and 4 (both left and outboard right hand engine). Rosaviatsia is investigating the occurrence together with Boeing and General Electric, the FAA and EASA have been notified about this very serious incident.

Unquote

Incident seems very interesting: the described jump in (apparent) temperature could well point to ICE CRYSTAL ICING.
The remark of "ice on unheated engine parts" and "unpredictable and poorly understood meteorological situations"also seem to point in this direction.
Would be a first case of BIG engines being so influenced, so far smaller (business jet types) have been influenced.
It does seem to indicate GE's are indeed a bit touchier than P&W with regards to icing matters.

[Harry O]

More on the incident at 41,000ft
Three engines damaged during 747-8F ice encounter (http://www.flightglobal.com/news/articles/three-engines-damaged-during-747-8f-ice-encounter-389763/)

[fenland787]

This describes it:

SKYbrary - High Level Ice Crystal Icing: Effects on Engines (http://www.skybrary.aero/index.php/High_Level_Ice_Crystal_Icing:_Effects_on_Engines)

[tdracer]

Incident seems very interesting: the described jump in (apparent) temperature could well point to ICE CRYSTAL ICING.
The remark of "ice on unheated engine parts" and "unpredictable and poorly understood meteorological situations"also seem to point in this direction.
Would be a first case of BIG engines being so influenced, so far smaller (business jet types) have been influenced.
It does seem to indicate GE's are indeed a bit touchier than P&W with regards to icing matters.

This is definitely being investigated as ice crystal icing, and I can reiterate that it's a "poorly understood meteorological situation".

Other 'big' engines have been affected by ice crystal icing - the CF6-80C2 has had a significant number of events and the PW2000 has had some recent events. However the recent 747-8 event is the first known event where significant engine damage resulted.

[golfyankeesierra]

Wow, 20 degrees warmer at F410 (@4 ft per degree per 1000ft) would take more then 3000 ft of your performance. That at F410 (!) together with eng fail plus surge, must have some scary sh*t....

[tdracer]

In most cases, the air temperature isn't really increasing in ice crystals. What actually happens is that the aircraft TAT probes are not designed to deal with ice crystals, and the TAT measurements get corrupted (TAT corruption is included in the AFM as an indication that ice crystal icing is present).

[golfyankeesierra]

TAT corruption is included in the AFM as an indication that ice crystal icing is present
Thanks tdracer, interesting info.

[lomapaseo]

Take the sensors and the damage may go away

[tdracer]

Take the sensors and the damage may go away


Huh?

Would you mind translating?

[StormyKnight]

TAT = Total air temperature - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Total_air_temperature)
In aviation, stagnation temperature is known as total air temperature and is measured by a temperature probe mounted on the surface of the aircraft. The probe is designed to bring the air to rest relative to the aircraft.

i.e. its is not designed to take into account the temperature effects of ice formation....

Thus the aircraft engine receives incorrect TAT measurements which can effect its operation including producing engine surges since its trying to operate using the measure TAT value rather than the actual effective TAT value.

Incorrect TAT values probably relates to incorrect calculated air induction volumes which leads to incorrectly calculated air/fuel mixtures.

[StormyKnight]

Huh?

Would you mind translating?

Without the incorrect TAT reading the engine would not have caused its own damage to itself....

[lomapaseo]

Take the sensors out and the damage may go away

laptop keyboard problem :)

If it's ice crystals they need to build up somewhere in order to cause downstream compressor damage. Internal probes may have served as a catcher for this very fast growth.

The operative word is may, lets see what the investigators say. I would hope that performance analysis could distinguish the cause effect vs time stamp.

[tdracer]

Without the incorrect TAT reading the engine would not have caused its own damage to itself

OK, now I understand the statement. It's inherently wrong, but I understand....

The engine control has multiple inputs of TAT. It gets the aircraft values of TAT, but it's primary indication is via an inlet mounted temperature probe. During 'normal' operation, the engine will use the aircraft TAT value (assuming it agrees - within a tolerance - of the engine probe). That way, minor differences in sensor tolerances won't result in cross-wing differences in engine operation.

But, if there is a significant difference between aircraft and engine sensors, the engine control will default to the engine sensors. The unheated dedicated engine temp sensor is not susceptible to ice crystal icing (pretty much, by definition, ice crystals will not accrete on sub-freezing surfaces - they just bounce off). So as soon as aircraft TAT was corrupted by ice crystal icing, the engine controls reverted to their dedicated sensors which were not significantly affected by ice crystals.

The engine damage was not cause by corrupted aircraft TAT measurements because the engine control wouldn't have used it.

Readers Digest version - we design for this.

[tdracer]

If it's ice crystals they need to build up somewhere in order to cause downstream compressor damage. Internal probes may have served as a catcher for this very fast growth.

Interesting - I was writing a reply while you were giving a slightly "different" question.

Internal probe icing has contributed to downstream damage on other engines. However the 747-8 (and similar 787) GE engines incorporate probe heat basic on the internal engine probes that may be subject to icing.

[737er]

So really, concentrating on a TAT probe design resistant to ice crystals may be the best solution?

[tdracer]

So really, concentrating on a TAT probe design resistant to ice crystals may be the best solution?

The TAT probe icing is a symptom, not the real problem. The real problem is the ice crystals form ice internal to the engine that can disrupt normal engine operation.