Doubling the number of aircraft will affect us all
By Jim Sparks
Programmed correction factor
Other concerns with doors are air leaks. Should a small area leak occur in a door seal of an aircraft that is pressurized, the velocity of the air through the leak will be significant. High velocity airflow through a small opening has an impact on the perpendicular airflow across the outer skin and subsequently impacts the static ports.
Once all the potential airflow hazards are corrected, or at least found to be in tolerance, attention should then be paid to the Air Data Computer (ADC). Most have a Static Defect Correction Module installed. This is a device that is specific to each aircraft type and provides a programmed correction factor calculated by the airframe manufacturer during aircraft certification taking into account aerodynamic and mechanical deficiencies in the static pressure system. In many cases aircraft that plan to operate in accordance with the new specifications will have to have a new Static Source Error Correction Module installed within the ADC. This means that ADCs that are the same basic type are not always interchangeable between different types of aircraft. In fact, in most Airworthiness documents, specific part number ADCs are called
out for RVSM aircraft.
Ongoing maintenanceand testing
Once the various maintenance procedures
are complied with, the completed RVSM application is submitted to the CMO along with an RVSM operating manual. Included within are the stipulations for ongoing maintenance as well as tools and equipment that must be used to validate the various operations. This will include a procedure where a flight crewmember has to conduct a visual check of the airflow path to the static ports prior to flight in RVSM airspace. If the inspection is not properly acknowledged for the day of RVSM flight, a financial penalty may be assessed.
Once the data package is accepted by the CMO, the aircraft may be obliged to fly a demonstration flight. This is completed by either installing a portable differential global positioning sensor or Ground Monitoring Unit (GMU) on board or conducting a Height Monitoring Unit (HMU) flight and is accomplished by flying over the HMU station located at Strumble in the United Kingdom. In the event the aircraft is part of a group, and adequate data has already been obtained, the test flight may be waived.
Ongoing maintenance is an issue all in itself. Should an unfamiliar aircraft arrive at a service center, one of the most
important questions asked should be about RVSM qualification. For example, if altimeter certification is due per airworthiness requirements, the air data computers are to be removed and sent out and replacement units are to be installed. Research needs to be conducted to determine RVSM acceptable part numbers. Installation of one part number may be valid for the basic aircraft, but not for RVSM flight.
Any work on auto flight systems should be followed with operational checks listed in the specific aircraft RVSM operating manual. Any damage that may impact static error such as fuselage skin repairs forward of the static ports may require re-evaluation. The Minimum Equipment List will also be influenced in many cases. In the event of an ADC failure, many aircraft can still operate by using a reversion mode and use the single operating unit to supply both pilot and co-pilot with altitude data. This is still acceptable as long as the flight does not require RVSM.
RVSM is on our doorstep. The ability of aircraft both old and new to perform to new standards falls into the hands of those of us in the maintenance business. It's up to us to exhibit additional
awareness and verify that even the most complex aircraft can safely and legally transport its payload with considerably less vertical margin of error.