View Full Version : St. Elmos Fire - side effects?

8th Dec 2009, 23:15
We operate a Hawker 900 - Proline 21 and Honeywell TFE 731-50R engines. The aircraft is a year old, barely 300 hrs since new and all running swell 'till a couple of weeks ago.

Following a night departure in vicinity of active CB's and significant cumulus cloud cover, we experienced several displays of St. Elmos fire during the climb. Spectacular, but not unusual. Thirty mins into the cruise however, something strange was indicated on the left engine - significantly higher than normal ITT, 10% more fuel flow than the other engine, and an increase in N2 of approx 2% more than usual. All was reported, DEECS were downloaded and data sent to Honeywell - nothing unusual showed up, and next flight gave all normal and balanced indications - end of story - or so we thought.

Ten or so flights later we departed in similar conditions (except that no CB active in the vicinity). Same display of St. Elmo's fire, and all indications normal. I thought to myself (jokingly), here we go again, but nothing - until we got to top of climb, when EXACTLY the same indications appeared. High ITT, high N2 and High fuel flow left engine.

Any ideas anyone, cos Honeywell sure don't have? BTW, fuel flow indications were certainly correct due to REAL imbalance caused, but as for other indications, who knows? Last DEEC down load showed nothing unusual. Is it possible that static electricity/differences could be the cause of this/indications/engine computers...?

8th Dec 2009, 23:43
I'd look into the possibility of a badly shielded wire allowing a static charge to build up and affect the EEC inputs.

Since it only affects 1 engine, there's likely something wrong with the control electronics.

9th Dec 2009, 01:36
Engine manufacturers should be able to trend monitor the data to determine exactly what part of the engine is causing the shift. If it's in the front end where St Elmo's fire can be seen then it's likley blade tip rubbing. But I'll leave it to the trend monitoring experts.

9th Dec 2009, 02:41
In my experience of these types of problems, the source is more often at the front of the aircraft.
Check if the flight deck glareshield or instrument panel is made of a metal composite or covered in metalized paint. If so then these require good bonding as the static from the windscreen can be transmitted to the instrument panel and thence the instruments. Check for corrosion under the bonding strips.
Obviously, check the windscreen bonding which probably ties together the two main panels with the fuselage and perhaps should tie in with the glareshield / instrument panels. A check of the windscreen heating circuits might also be worth doing.
The left handedness of the problem might be a clue; IIRC the 125 wiring scheme separates the digital buses between left and right side of the aircraft to provide some failure proof segregation.
A more obscure fault might be with the bonding of the main entry door. As implausible as this sounds, the door, located on left hand side, completes the fuselage’s electrical cage. As an example of electrical interference, some older HF installations would ‘ring the fire bell’ or disturb the OAT gauge when transmitting on the ground with the door open, but otherwise were OK in flight.

9th Dec 2009, 11:43
lomapaseo...thanks, but Honeywell looked at the DEEC downlaods and said nothing abnormal showed up.

safetypee..even more thanks. We did have a screen overheat pop up a few flights ago. It was left main, and right sidescreen, so maybe you are onto something here. What I find most peculiar though, is that while it is possible that some of the engine indications may be indication faults, the increased fuel flow indication was genuine. The left engine WAS using more fuel. I will pass on your thoughts to our tech rep, if you have any more please let me know.

9th Dec 2009, 15:27
Bleed valves? Engine Anti-icing valve stuck open one side?

9th Dec 2009, 20:35
Considering any ‘real’ changes to the engine vice false indications, still start the search at the front of the aircraft and work back.
Is the engine anti-ice switching in the overhead panel (left side?), or on a sub panel next to the windshield – I assume that the 900 is similar to previous 125 variants.
I don’t know about the particular 900 engine installation, but some other aircraft types had auto engine anti-ice activation as part of the anti-surge protection, but I can’t recall any 125s being fitted. Thus, although this is unlikely to apply, this or a similar problem may result in the anti-icing being auto-activated at altitude with thrust reduction, i.e. top of climb. Check thrust lever quadrant switches.
Consider air cond valves hi/lo auto switching. However, with failing memory, this may only apply to older 125 variants, and electrical activation is still required – St Elmos fire is the key. Are the air valves switched for the left side of the overhead panel? If so back to bonding and stray electrical paths / faulty switches (subject to shorting in high static). I knew of one aircraft where the autopilot would disconnected anywhere near a Cb.
Electrical problems may also depend on who / where the completion was carried out. Older 125s were predominantly A or B models, with greater variability in the A (USA/FAA) market.

30th Dec 2009, 22:09
Let me guess, it has not done it again???

30th Dec 2009, 22:57
St Elmo is unlikely to cause internal engine damage BUT lightning strikes on props can and do damage bearings. Evidently arcing through the bearings can pit the bearing races, balls, rollers which may then begin to deteriorate to subsequent failure.

Does anyone have a handle on how extensive this problem may be ?

How many pilots record a prop lightning strike.?

non iron
31st Dec 2009, 06:33
l remember years ago listening to one of the ndb`s tuned to Radio 4 whilst in bumpiness.
lt was from a live transmission of ghostly activity from a north country aviation museum.
All the noises were there, as they explained. My crisp packet popped - we were unpressurised.
And then l heard their description, what`s that ? They had never heard that noise before, like being pelted with pebbles.

l had. 2 minutes before with the props shedding ice.

Just give the engineers a clue, keep calm and continue.

Would you want them questioning your competence ?

31st Dec 2009, 06:50
I think Intruder may have it. Poor bonding/shielding with HIRF or RF fields play havoc with other systems / indications if bonding and or shielding is poor.

We nearly had to set down in the Iranian boondocks many moons ago with a dodgy engine that turned out to only lose oil pressure etc with HF transmissions:uhoh:. Lucky the travelling ginger beer was a good radio bloke:hmm:
It can be a bugger to find offending wire / bonding point though.

non iron
31st Dec 2009, 07:32
Sorry Amp, wrong place.