King Air B300 Structural Damage

[SKA350ENG]

Hi. I am looking for some information. I have seen two King Air B300's, a 400 series and a 600 series. These aircraft have relatively low hours and they are showing some very unusual structural damage around the LH engine firewall, This damage is in the form of buckling of the stainless steel firewall, inboard side predominately. More severe buckling and deformation of the first two bulkheads just aft of the firewall, inboard side.

The most severe damage is located inside the LH nacelle fuel tank with deformation and broken 'z' stringers just aft of where it connects to the firewall.

The aircraft have not been 'heavy landed' and the propellers, engine frame and undercarriage show no signs of damage or contact due to a heavy landing event. To correspond with the damage I'm seeing it would have hit the ground and stayed there.

I believe the buckling is secondary damage, probably from turbulence or a firm landing, but within the certification basis (1.5G-2G.

The start of the failure may have been an over pressurization event in the nacelle fuel tank. But how? No signs of electrical failure i.e arcing leading to a contained explosion.

Has anyone seen damage like this or damage that appears to be an over pressurization in the nacelle fuel tank?

I have heard rumblings of nacelle over pressurization that has prevented under carriage from being retracted and locked up. But these can not be confirmed, yet. I have spoken to some contacts in Australia who operate B200 and B300 fleets. Nothing helpful, yet.

If anyone can point me towards any reports or contacts so I can look at similar causal factors or issues I would really appreciate it. I need to have a idea of what is causing it as it is unusual.

 

[Compositepro]

This just a wild guess but have you considered that the skin is expanding because of heat, and it buckles because it is being restrained by a cold, stiff frame.

 

[MiketheEngineer]

Have you considered contacting the mfg. I believe that under the FAA rules - you must do that??!!

 

[thruthefence]

Have you checked the SDR (service difficulty database)?

http://av-info.faa.gov/sdrx/

You'll need the p/n's of the damaged parts.

Been awhile, and maybe the 300's are different from my old kingair experience, but I thought the nacelle tank was a bladder type?

 

[SKA350ENG]

I have attached some photo's as it is not heat.

HBC have been contacted and the first aircraft's repair schedule was raised by them.

The nacelle tank has a liner that holds the fuel, but the nacelle tank is the structure.

 

[berkshire]

ska350eng
At the very least you should submit a malfunction and difficulty report.

http://av-info.faa.gov/sdrx/SubmissionsGeneralAviation.aspx

B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[Compositepro]

It does, indeed, look like an over-pressurized fuel tank. The causes I can think of are:

A plugged or restricted vent line while filling.

Overfilling the tank, where fuel could not flow out the vent as fast as it was pumped in.

A small ignition in the tank from static.

 

[thruthefence]

These aircraft are not single point. They are fueled "overwing", or in this case, "over nacelle", just like your automobile. There's no way overly aggressive line personnel can overpressure the fuel cell.

The tank is vented through a check valve, atmospheric pressure in to the tank. I don't believe there are provisions for venting excessive pressure.

There is an electric transfer pump that feeds the nacelle tank from the "stub wing", (that area between the nacelle & fuselage) that operates automatically, from a float switch (or an analogous solid state device) in the nacelle tank. It should not operate, until the level in the nacelle tank reaches a certain point.(the engine draws all it's fuel from the nacelle tank) I seem to recall the output of this pump is around 5 -7 psi, which I bet is more then enough to damage the structure, especially if the engine is at a low power setting, or not running at all, ie running a check list after a "top off" refueling.

As I said, the process is automatic, with no crew input. The only annunciation the crew would get,(again, as I recall) is "no transfer" after the fuel is totally pumped out of the stub wing (or "aux") tank. By this time, the damage to the structure is done.

 

[vonsteimel]

Sounds like its time for a test flight if you ask me....

 

[MiketheEngineer]

Not by me!!!

 

[dgapilot]

Looks like a hard landing or some other form of overstress - excessive gust loads. I don't think it has anything to do with the fuel tank. By any chance did this aircraft have a prop strike in it's history? Looks like the forces were up and outboard. A prop strike would give that type of load.

 

[SKA350ENG]

Not a hard landing. No prop damage due strike or history of one.

Given the level and direction of damage this was my first though. When you look at the damage it would have been a very 'firm' landing. There would have been a prop strike, undercarriage strut damage including mounting points, and the tyres would have separated from the rims or at the very least scrub marks on the tyres. Nothing on either side. The OEM even 'suggested' that it was left hand contact first. If this was the case those areas would definitely have had signs of damage.

Structural specialists have surveyed this damage and they believe that it is an area of secondary damage which is why we are looking at a failure of the Nacelle tank structure leading to a weakening and damage similar to a heavy landing.

Nacelle tank over pressurising is a plausible cause. I believe there was an issue with a maintenance error to do with incorrect fitment of the check valve in the tank. Anyone heard/know of this?

 

[jd750ace]

This is a common type of damage on the King Air Series, and is, indeed, from over-pressurization of the tank. There is, if I remember correctly, a section in the Super King Air Series SIRM that gives a reccomended parts list for items to replace when this occurs.

Hope this helps. If you can ever manage to get the phone number at Hawker Beechcraft of Steve Stitcher, he's a wealth of knowledge on King Air airframes.

 

[jd750ace]

My error. I just viewed the images. It looks like someone has attempted to support the area from the bottom of the firewall, which is not smart! If you can go into the gear well and run the bore scope forward in the channels where the truss bolt barrel nuts are located, check for the fasteners working in that area. It is hard to get to on the upper one, given the tank liner, but the lower ones are noted for loosening up in higher time 200's. I've seen them like that in aircraft with as few as 6,0000 hours.

 

[pdlugen]

Have you checked to see if the nacelle vent check valve is installed properly or opening at the correct psi, which is around 6 psi?