When an aircraft manufacturer modifies an airplane, the company must apply to the Federal Aviation Administration Aircraft Certification Office to verify that the design is structurally and mechanically sound. In reviewing the design, FAA engineers look carefully at the structural analysis used by the manufacturer in order to make sure the analysis results are correct. They must verify, for example, that the wall strength of high-pressure tubes in a modified engine fall within a specified margin of safety, or that the floor of an aircraft, carrying greater loads, has the rigidity required to meet FAA standards. In the past, verification has meant searching through handbooks, such as the Roark "Formulas for Stress and Strain," to find a section that applies to the analysis, selecting a series of formulas, generating equations and then solving them with a calculator. But referring to a handbook and performing hand calculations is so time-consuming that FAA engineers have been unable to efficiently verify the analyses that have been submitted to them. Recently, they have reduced the time required to verify manufacturers' analyses by using a computerized engineering handbook called The Desktop Engineer from Desktop Engineering Int'l Inc., Woodcliff Lake, New Jersey, which automates the tasks of finding the relevant material and performing the necessary calculations. The time savings provided by the handbook makes it possible to check analyses more thoroughly which provides greater assurance that the designs are structurally adequate.

The Federal Aviation Administration's Aircraft Certification Office, based in Valley Stream, New York, certifies manufacturers' plans for modifications of existing aircraft, both commercial and private. Some of the main types of modifications involve adding weight to the aircraft, such as adding passenger seats, an auxiliary fuel tank, or electronic equipment. Another common modification is changing the engine size. The aircraft manufacturer submits an application for certification for the modification. On minor modifications, the manufacturer prepares the application and presents the calculations to support the design. For a major design change, the manufacturer will hire a consultant, a Designated Engineering Representative (DER) with credentials from the FAA, to do most of the analysis and submit it to us for review. Most of the analysis work that is commissioned by the office is done by hand, though some of the more sophisticated DER's use computers.

One of the more common types of analysis that has to be verified by the Aircraft Certification Office is the analysis of pressure vessels, such as the high-pressure tubes in an aircraft's engine. When the engine is modified, different tubes for bleed air often have to be installed. The pressure varies so we have to determine the minimum wall thickness that will provide adequate wall strength with a specified margin of safety. In the past, a hand calculation analysis of pressure vessels has taken as much as a few hours. But now we solve the problem with the computerized handbook in less than five minutes, using the "Thin-Walled Pressure Vessel" module of the program. The procedure simply involves entering the height, diameter and thickness of the tubes, the type of material, and specifying internal pressure as the loading condition. Based on these values, the program quickly determines whether the tubes can withstand the pressure within the guidelines.

Often, the manufacturer or DER uses finite element analysis (FEA) for the structural analysis of the aircraft modification. Because of the many calculations involved in FEA, it can take us several days of tedious work to verify this type of analysis with hand calculations. Now, with a computerized engineering handbook, it generally takes less than an hour. A good example is verifying a structure involving multispan beams in the floor of an aircraft. This is often used when a manufacturer wants to add more seats or an auxiliary fuel tank to the floor. To determine the impact of the larger loads on the floor, the manufacturer performs FEA by using long multispan beam elements to represent the floor and applies a loading condition for each seat and piece of equipment mounted to it. To verify that the overall rigidity of the structure meets FAA standards, engineers use the super-positioning module of the handbook, which lets them look at the effects of several concurrent loads acting on the composite floor structure. They first determine the properties of the beam's individual elements. Then they go to the handbook's beam and column module, select multispan beams, and choose the number and location of supports. They then apply the load cases one by one. The output for each load case, consisting of loads and boundary moments is automatically saved by the program as it is generated. With the properties of all the beam's individual elements determined, they combine the output files in the superposition module, which calculates the whole structure's rigidity. The propensity of the beam to twist can also be included in these calculations by using the "torsional module." The super-positioning module of the handbook makes it possible to evaluate virtually any beam-like structure regardless of its complexity. Another advantage is that documentation of all the calculations can be produced with just a few keystrokes. They can ask the program to print either a summary or a more detailed report, which, in the case of the multispan beam analysis, would give deflections, forces and stresses at different points on the beam. They can also have the program calculate and print required geometric section properties.

By being able to check manufacturers' analyses more thoroughly than with hand calculations, FAA engineers have uncovered mistakes in the applicants' calculations that probably would not have been found by manual methods. One of the more critical cases involved a complicated analysis of a floor beam. To verify that the design was adequate with manual methods, they probably would have checked only the more important calculations because checking each one by hand would have been prohibitively time-consuming. With the computerized engineering handbook they completely checked every single one of the calculations in about 30 minutes. The beam analysis quickly gave them shear loads, reaction loads and bending moments, along with a shear diagram and bending moments diagram, which showed them graphically where the maximum moments were. The engineers could see at a glance that several of the bending moments supplied by the applicant were too low. The low bending moments would have resulted in a floor design with significantly less strength than was needed under the new loading conditions. The engineers advised the applicant of the correct bending moments and will use the handbook to review the revised design. The Desktop Engineer has significantly increased efficiency in verifying manufacturers' analyses and automating the time-consuming tasks of looking up information in a handbook and manually calculating the equations. Not only has the time required to solve problems been reduced, so has the chance of making mistakes. The verification is now complete, providing assurance that the manufacturer's aircraft designs are structurally adequate. The handbook has also been found to be very easy to use and highly interactive. The staff have become proficient in the program quite fast, without much training. The computerized engineering handbook is interactive and has automatic input verification and other aids to prevent misunderstanding and help make the problem-solving process correct as well as efficient. Furthermore, the program performs equation substitutions, integration, and boundary condition analysis in exactly the way an engineer would solve the problem by hand.

The Desktop Engineer is a computerized engineering handbook that provides over 5000 solutions to common engineering applications found in over 100 engineering reference books. It includes over 50 modules grouped into the following categories: geometric analysis; static analysis; dynamic analysis and buckling analysis. These categories are used to analysis structures including straight beams, curved beams, cables, circular arches, circular rings, columns, discrete systems, disks, foundations, frames, grillages, helical springs, plates, shafts, shells and solids. Thin Walled Sections is an important optional new module of The Desktop Engineer. This module calculates more than 30 nominal section properties, including twelve torsional properties, for arbitrary thin-walled sections. All modules are self-prompting to help the user find section properties, displacement, forces, stresses, etc. The Desktop Engineer also includes a material property database and a unit conversion utility.

For more information contact Desktop Engineering Int'l Inc., 172 Brodway, Woodcliff Lake NJ 07677. Phone: 800-888-8680 or 201-505-9200 Fax: 201-505-1566