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    DC9/MD80/737 Airstairs: The forgotten ones

    Sept. 1, 2000

    DC9/MD80/737 Airstairs

    The forgotten ones

    By Greg Napert

    September 2000

    There was a time when the image of a passenger aircraft pulling up to a small terminal and deploying a futuristic-looking set of stairs was a common sight at an airport. For corporate and general aviation aircraft, it still is. But as passenger terminals developed and terminal Jetways and mobile stairways became commonplace; the need for airstairs became greatly diminished.
    Aircraft such as the DC9, MD80, MD90, and 737 aircraft have had a long enough history that many of them were built with the forward passenger air stair assembly. As the need for them was reduced, later models were produced without forward airstairs. Many aircraft were also converted to non-airstair versions Ñ a 200+ lb. weight savings for the operator.
    Regardless of the history, there continues to be a need for airstairs for specific operations. Freight operators, corporate conversions, and newer corporate variations of these aircraft such as the 717 still find air stair installations a necessity. The result is that there is still a need for maintenance and repair knowledge, replacement parts and qualified sources for overhaul.

    Basic airstair description
    The airstair installation, with some variations from aircraft to aircraft, includes four main components:
    1. Forward and aft rail assembly
    2. Carriage assembly
    3. Electrical installations
    4. Airstair assembly
    The airstair assembly and the airstair door are powered with 115/200 VAC, 400 Hz, 3-phase power source located on the aircraft. The airstair installation and the airstair door may also be operated with a power source of 24 to 28 VDC.
    Airstair assembly consists of two rigid beams connected to the ladder assembly, which also consists of the steps, hand-rails, lights, and wheels that allow the airstairs to seat firmly on the ground when extended. This is what extends from the carriage assembly to the ground for entering or exiting the aircraft.

    Servicing airstairs
    Light bulbs below each stair tread should be checked frequently and replaced, if necessary.

    One of the few facilities in the country that offer air stair overhaul services is Professional Aircraft Accessories. According to Joe Gramzinski, Chief Inspector for Professional Aircraft Accessories in Titusville, FL, "We have been servicing, repairing and overhauling airstairs that are installed on various aircraft including the DC-9, MD-80, MD-90, along with the 717 and 737 for over 10 years. Although some aspects of the overhaul can be a challenge, we can usually overhaul a standard airstair assembly within 30 days. This includes installing it on our universal test stand, this will allow us to give it a operational check and a complete visual inspection prior to teardown."
    Of the many challenges related to overhauling some of this seldom used and often abused equipment, corrosion is a major factor in determining the overhaul requirements. Airstairs are exposed to the elements and experience a considerable amount of cracking due to widely varying stress and weight loads on the steps.

    The process Billy McLean, Shop Supervisor for PAA explains that the overhaul process can be somewhat involved, but it is interesting to note the wide variety of conditions that some of these stairs are in. "Most stairs we receive don't even operate when we place them on the test stand." McLean explains the overhaul process starts with an incoming inspection and test to establish the received condition of the unit. Next, the cover is removed and the stairs are removed, and the carriage assembly will be detached thus allowing further inspection and component removal from these assemblies. The actuator assembly will be removed and overhauled separately per the manufacturer's overhaul manual. The stairs are quite big and bulky in size so these are disassembled in sections as needed. The carriage assembly will also be disassembled, cleaned, inspected, chains and sprockets replaced if needed, cam followers, etc.
    Intergranular corrosion can result in cracking and deterioration of airstair assemble. Inspec carefully for signs of corrosion.
    Field repairs must be evaluated according to overhaul manual and supporting documentation

    The forward and aft rails along with the handrails must also be disassembled, cleaned, inspected, parts and cables replaced if needed.
    After all of these assemblies have been serviced and repaired, they will all be primed and painted (if applicable). New non-skid material is replaced on the stair steps. This material is simply a stick-on adhesive that is pressed in position and then sealed with a sealant around the edges. This sealant prevents the non-skid material from pealing and prevents water from getting under and causing corrosion.
    The unit is then assembled completely to the manufacturer's specifications for final testing and rigging, then shipped to the customers worldwide for installation onto their aircraft or on their shelf as inventory.

    Real life According to McLean, "There aren't a long list of mandatory parts that have to be replaced at each overhaul, but there are a few. There are also some components that we have to NDI as well, but for the most part, this is not a 'safety of flight' component, so there are not many critical inspections." Much of the rebuild simply involves a great deal of degreasing, cleaning, scraping and repainting. It's a great deal of physical labor "elbow grease" that's required. You also need the proper tools to do a good job. An airstair stand is used at PAA to duplicate the mounting studs that are on the aircraft installation. The stand simulates the exact height of the aircraft so as to be able to test whether it's adjusting properly to meet the ground. Darryl Stazenski, Inspector for PAA, says "There are certain areas that are problematic in terms of cracking or corrosion and we look at those areas with a bit more discretion. We often find field repairs that are questionable. We also see good repairs, but these have to be inspected especially close to be sure they conform to the overhaul manual." McLean adds, "Unfortunately, the biggest problems surrounding repair/overhaul of these airstairs is lack of parts support and repair information, and the high price tag for replacement parts. Additionally, some of the lower level assembly parts aren't available. The result is that instead of being able to replace a small stud or piece of simple hardware, we're forced to purchase an entire assembly of related parts." A good example is of studs that are positioned on the lower tray assembly that are bonded to the composite structure. These studs could easily be repaired by performing a bit of composite work, but the studs aren't available, so if you have a damaged stud, you've got to replace the entire lower tray assembly. "There is no data available from Douglas/ Boeing to make this repair, so we're stuck with this scenario in several instances. What operators choose to do in some of these cases is salvage components from other serviceable stairs. This is often a much more cost-effective solution, but it is not always available," he explains. McLean says that one thing that can hold up an overhaul is missing components that were removed from the stairs by the operator and not turned in for overhaul. "We usually call the customer to ask if they have the part, and if they do, they either have to send it to us to inspect so we can sign off the overhaul, or make a record of the fact that the customer is holding the part. If that happens, we will approve all repairs with the exception of that component."
    One example of a component that can be problematic is the round access panel on the lower tray. This panel is a fiberglass cover piece of round aluminum, and many customers don't ship it with the stairs. "Unfortunately, if it is lost, we can't manufacture another, and a new one costs over $6,000 to replace," explains McLean. "Yet another is the locking mechanism (hooks) that is used to hold the stairway in place when the stairs are fully extended. These hooks can cost over $3,000 to replace."
    In terms of the airstair actuator mechanism, McLean says, "We have occasionally replaced the chains and sprockets, but they are normally in pretty good condition. They have a tensioner on them that accounts for a considerable amount of wear. You can usually tell when they are stretched beyond limits due to the carriage binding and when adjustments are difficult to make. The chains are actually one of the more rugged components in the system, however. More than likely, a good cleaning, inspection, and lubrication are all that's required for these components in order to return it to service." Common problems One of the more common items that require a considerable amount of rework are the upper and lower enclosure assemblies. According to McLean, "There are wear strips that are located on the top enclosure where the stairs rub as they are retracted and extended. These wear strips will wear occasionally to the point that the edges stick up and if they catch on the stair assembly, they will peel the strips off of the cover and possibly cause binding." He continues, "Another occasional problem as well is the stops of the rear of the airstair that shut down the motors when it is retracted will also be damaged." On the stair that was in the shop during this story, the stops were completely broken off and the stair was hitting the back of the enclosure. This force alone was enough to shut down the stair. Unbeknownst to the operator, however, the stair would have eventually busted through the enclosure and then been jammed in place," he says. Vibration of the stair assembly is probably the biggest detriment to the entire assembly, second to corrosion. According to Stazenski, "We find many of the rivets and hardware items worn through or loose. Corrosion and delamination is definitely the biggest problem."
    Stazenski adds, "For the stair assembly we were looking at, you have the stair sides delaminating, major corrosion, a lot of times you have cracks on the steps, etc. The delamination can be repaired, most of the time, by re-laminating the layers with adhesive and pressure. Corrosion is addressed in various manners to include sanding, scraping, chemical treating, and replacing components, depending on the extend of the corrosion." Options for expensive parts The volume of repair and overhaul of airstair assemblies really don't justify manufacturing aftermarket parts for them. Yet the OEM parts can seem unjustifiably expensive, so PAA uses another option that is available.
    "There are so many units that are available that we can often salvage what we need from units that are not in service," says Stazenski. He explains that many of the DC9s that are equipped with these stairs are no longer used for passenger service and are being use for freight. There are also hundreds of airstair assemblies that are installed in aircraft that are not being utilized.
    "We currently own three sets of airstairs that we are salvaging parts from to rebuild units," says Stazenski. "Keep in mind that the customer has the option of going with all new parts if they want to pay the price, but at least we give them the option of using serviceable parts for non-critical components."

    How long does it take?
    Depending on the situation, PAA can completely overhaul an assembly in five or six days. In reality, however, it takes a bit longer to process due to parts availability and work load. PAA ends up promising a 30-day turnaround period.
    "The best scenario for our customers, and ourselves," claims Stazenski, "is to have our customers plan ahead and have at least one airstair assembly that is overhauled for AOG purposes. Then you never really end up with an AOG or rush situation because you use the overhauled stair when you need it, and send in the backup for overhaul without putting a rush on it. In reality, what happens is we receive three at a time in and they're all AOG."
    He continues, "One thing that the airline customers are pretty good at is lubricating the track and gear assemblies. We very rarely find one that is dry. Also, the lubrication really doesn't have much of a chance to wash off - it's basically covered at most times on the top and bottom of the mechanism. We do find some interesting items inside the assembly between the top and bottom cover. Typically, these items, such as pens, hair pins, etc. fall into or are left on a stair and are pulled into the trays when the stair is retracted."

    Basic advice
    "In general, it's interesting how some of the more basic maintenance is not done on these steps," explains Stazenski. "Simple, yet critical items such as the lights that are on the bottom of each step for lighting the stairway are often burned out or missing. Very rarely do we find them all operational. We simply replace all of the bulbs with new to make sure that the lighting lasts for a while. It doesn't take much to keep these stairs operational and a little care can go a long way towards reducing costs related to overhaul."