Pneumatic De-ice Boots
Inspection and maintenance tips
By Joe Escobar
It is that time of year again when the aircraft we work on will be subjected to icing conditions. In order to keep those aircraft flying safely, their de-icing systems need to be maintained at an efficient level. In this article, we will discuss some of the issues pertaining to pneumatic de-ice boot inspection and maintenance as well as some removal and installation tips.
De-ice boots prevent the accumulation of ice by breaking it up at the leading edge surfaces. A typical system uses air pressure that is generated from either an air pump driven by the engine, or by bleed air from the engine. From there, it goes through a pressure regulator and then a flow control valve, which is sequenced off of a timer. The timer can either be operated manually by the pilot or in automatic cycle where it will cycle every one to three minutes.
De-ice boots should be inspected routinely as per the manufacturer’s recommendations. Regular cleaning of the boots is helpful not only for appearance sake, but for enabling the detection of defects in the boot. General condition of the boots should be evaluated and any damaged areas repaired as required.
Cuts in the de-ice boots should be repaired as soon as possible, which will prevent the cut from enlarging during operation and will also prevent water from entering the boot. If water enters the de-ice boot, it can freeze and prevent the boot from operating properly.
Boots should also be inspected for blow holes. This defect looks like numerous pin holes on the boot. Richard Rauckhorst, Director of Engineering for Ice Shield De-Icing Systems, a product of B/E Aerospace, explains the phenomena. "The conductive edge sealer (bordering the de-ice boot) allows a path for static dissipation, and that is critical to the de-icer’s life. If you are flying in cold, dry air, you’ll build up a very large static discharge in that surface. And, you’ll actually get what we call ’blow holes’ in the surface of the part where you build up so much static, it has to dissipate. It’s just like a lightning bolt, a mini-lightning bolt. It will actually blow a hole in the surface of the part. So, that’s one of those repairs that you want to prevent. But to prevent it, you need to have a good conductive path."
As Rauckhorst mentions, maintaining a good conductive path is essential to de-ice boot life. The conductive edge sealer provides this path for static dissipation. Itis important to use only those products approved by the manufacturer. Substitution of an alternate sealer may seem harmless, but it may not provide the needed conduction forthe static to dissipate. Also, ensure that static dissipators like static wicks are maintained.
Patching the de-ice boot is a fairly simple procedure and typically involves using a template over the area and scuffing up the boot. It is then thoroughly cleaned with a solvent, and the repair patch is installed.
Patches are now available that have a pressure-sensitive adhesive on the back. The old process of having to apply glue layers to the boot and the patch are not necessary. After the area is scuffed and cleaned, the backing is taken off the patch and it is applied to the boot. Boot replacement should be considered in cases of excessive damage.
Before removing the old boot, ensure that the new boot received is the correct one, and that the air tube opening is placed correctly and the size is correct.
It is helpful to locate the centerline of the old boot at this time. The boot can be pulled back a little at both ends to visibly locate the line on the original boot. Then, using a water-soluble marker that won’t attack the paint, make a mark outboard of the boot on both ends. Another method might be to place a strip of tape in-line with the center mark. This saves a lot of time later on versus trying to re-establish a centerline.
When removing the old boot, it is easiest to cut the boot into thin strips to allow for easier removal.
"We recommend a utility knife with a hook-edged blade," explains Rauckhorst. "That allows you to cut strips into the de-icer 2 to 4 inches wide. A lot of people try to remove the boot all at one time, and that is fighting the impossible dream. There is just so much surface area that you are trying to pull off that aircraft. What we recommend is 2- to 4-inch strips going down span-wise of the boot, just cutting it all the way to the surface. The hook-edge knife protects the aircraft surface from scarring."
Extensive damage can be done by using razor blades or regular utility knives for this task. Even when using a hook-edged knife, be very careful not to damage the surface underneath.
Using a tie wrap to aid in removal of a trapped air pocket.
Once the boot has been taken off, the old adhesive needs to be removed. This can be a time-consuming task. Rauckhorst offers a procedure that makes the task easier. Basically, barrier material is placed around the area where the boot was, with a longer piece hanging down off the lower surface. Then, an approved stripper is applied to the adhesive, and the lower piece of barrier paper is folded up over the leading edge and secured to the top side, forming a type of envelope for the stripper to sit in. This keeps it from flashing off quickly, retaining the active agent in the stripper. After 20 or 30 minutes, the stripper can be removed, usually only requiring a follow-up with a damp solvent rag to remove any remaining adhesive residue .
With the old adhesive removed, the surface needs to be inspected for any damaged areas or corrosion. Damaged areas like dents or cracks need to be repaired per the manufacturer’s requirements. All corrosion, no matter how minor, needs to be removed. If allowed to go untreated, minor corrosion can escalate into major structural rework the next time the boot is removed.
Wet wings require special attention. In some instances, a barrier adhesive or cement is used on the exterior seams. Make sure to refer to the manufacturer’s maintenance manual and reseal any areas that may have been affected during the stripping process. Fuel seeping through these seams will attack the adhesive bond of the boot, causing it to separate from the wing.
Once the surface of the aircraft is prepared, the adhesive can be applied. The adhesive should be mixed thoroughly, ensuring that it’s shelf-life is not expired. Using fresh adhesive makes the task much easier.
Apply the adhesive in two layers, letting the first layer dry before applying the second. It is important to pull a vacuum on the boot when applying the adhesive to ensure a nice smooth application. Without a vacuum, you will get surface irregularities in the boot from the two pieces separating. In fact, it’s a good practice to pull a vacuum during adhesive application as well as installation to ensure a smooth installation.
Rauckhorst shares a tip on marking the centerline. "What we normally do is apply the first coat of adhesive, allow it to dry, then snap a chalk line, a very light chalk line, followed up by a ballpoint pen. Draw that line across there before you put the second coat of adhesive on. That way, the center line is permanently trapped between those two coats of adhesive so that when you tack it up you don’t wipe it off as you install it."
When the adhesive on the aircraft surface and boot is completely dry, the boot is ready for installation. The boot can be rolled up from the ends to meet where the air connection area is located. This needs to be the first area glued down. Then the leading edge is tacked down matching the marked centerline on the skin with that on the boot.
The adhesive is activated by wiping it with a solvent-dampened rag. This should be done in small areas, usually no more than four feet at a time. Then, the boot is laid down and rolled on the surface to ensure a good adhesion.
Once the leading edge is tacked down, you can work back towards the trailing edge of the boot. It is easier to work at a 45-degree angle to the leading edge, never perpendicular to it. This allows for easier access to the adhesive to re-activate it as you work down the boot. If you stop perpendicular to the leading edge, you run the risk of having spots that are too dry or solvent-laden underneath the boot. The steps of re-activation, laying the boot down, and rolling it should be repeated until the whole boot is secured.
If you are patient and careful during installation, you will end up with a nice, smooth finished product. However, there may be an occasion where an air bubble will be trapped underneath the boot. Rauckhorst shared a tip on how to remove a trapped air bubble. "What you can do is take a tie wrap or a thin piece of plastic with a very blunt, and I want to stress blunt, leading edge to it. You can take it and dip it in some toluene. You can work your way from the trailing edge up to that air bubble, allowing a path for that air to escape. While the tie wrap is still in there, start removing it while working down that area at that time to remove any air that you may have trapped underneath the boot."
Once the boot is secured, any excess adhesive can be cleaned up from the surface and the conductive edge sealer applied. Again, follow the manufacturer’s recommendations on edge sealer application. Paint touch up should be done as required.
These tips should prove useful in maintaining pneumatic de-ice boots. Always follow manufacturer’s instructions when repairing or replacing boots. With proper care, they should provide years of trouble-free service on the aircraft.
Additional resources.... Goodrich Aerospace
De-Icing and Specialty Systems Division
1555 Corporate Woods Parkway
Uniontown, OH 44685-8799
Attn: Product Support
(800) DEICERS [800-334-2377]
www.deicingsystems.goodrich.com B/E Aerospace
Ice Shield De-Icing Systems™
WV Route 39, HC 79 Box 200
Fenwick, WV 26202