AERO2514 : Impact Air and Your Aircraft Fleet - Horizontal Tailplane - Engineering Assignment Help

Assignment Task :

Assignment brief 

The objective of this assignment is to produce a report analysing issues relevant to the  finding of cracks in the primary structure in an active civil aircraft fleet. Emphasis should be  given to the identification and analysis of the key deficiencies after taking guidance from the  various standards available. Since information is limited and imperfect, any assumptions  need to be stated and defended. Answers to the questions posed will need to include the  reasoning behind the decisions taken. 

Background scenario for the assignment 

You and your airworthiness team are the ASI managers for Impact Air and your aircraft fleet  is dominated by the ‘Gomad GA69’ aircraft 

Your airworthiness team oversees the airworthiness of the light twin turboprop utility  aircraft (<8600kg) the Gomad GA69. These are used for passenger (up to 18 PAX) and freight  services in Australian regional centres. Recently your servicing personnel have been finding  ‘small cracks’ (less than 20mm in length) in the web of the forward spar of the ‘all flying’  horizontal tailplane. These aircraft had between 1800 and 3800 flight hrs at the time that  the cracks were detected. The structure is a box with a forward and aft web. The forward  web has a large 4” lightening hole with a cold formed rim to aid in stiffness and is covered  with a 1mm thick sheet metal doubler, which is riveted on the 1mm thick sheet metal web  (aluminium alloy 7075-T6, painted with an epoxy primer). The cracking is being detected at  45o angles at four possible locations about the hole in the covered primary web, as shown in  Figure 1.

 

Figure 1: A schematic of the lightening hole in the forward spar web with the locations of  possible areas of cracking. While the origin areas of the cracks are uncertain inspection is to  be carried out on the surface of the web on the inside of the box structure from the edge to  about 1” away from the edge. 

The manufacture has supplied an inspection method that is visual and involves using a  mirror to inspect the surface of the web on the inside of the box structure from the edge to  about 1” away from the edge for the cracks. This inspection is to be applied every 100 flight  hrs. Notwithstanding the cracking, the manufacturer has indicated that the tail carry through structure should have a safe life of 30,000hrs under the design loads. The  manufacture has indicated that the cause of the cracking is the uncontrolled flapping of the  horizontal tail whilst on the ground when gust stops are not fitted. This impacting of the tail  on the stops is suspected to be causing crack development by ‘local tearing’ since they have  reported that some large cracks that they have inspected had evidence of tearing on the  fracture surfaces. These surfaces were very damaged by rubbing so nothing more was  reported. The manufacture has discounted that the crack could be due to fatigue crack  growth due to normal flight loads. Your team has been aware of this explanation for the  damage and has been diligent in fitting gust stops for your fleet in order to prevent this  damage. 

The manufacturer has also reported cracking in the adjacent horizontal tail plane  attachment brackets and in the upper and lower flanges of the box associated with the  attachment areas. The manufacture has designed a retrofit repair for these areas that is to  improve the durability of these locations since this cracking has been identified as fatigue.  The repair is expensive and time consuming to apply, consequently it has only been applied  to one of your fleet aircraft. It consists of doublers to be fitted about each of the mounting  points, new beefed up mounts and doublers to be fitted to the upper and lower flanges of  both the forward and rear spars. 

The manufacture has not fatigue tested the tailplane structure nor has the manufacture  developed a calibrated flight loads spectrum for the horizontal tail, arguing that the  tailplane was designed to be stronger than the wing and centre fuselage structure, which  had been fatigue tested in a full-scale fatigue test, and been found to have a safe life of at  least 30000 hrs at a scatter factor of 3.3. (some parts were found to crack earlier than this  and these are to be replaced when they reach their shorter safe lives). 

While flight loads had been measured for the wing and centre fuselage for use in the fatigue  test, no flight loads program has been undertaken for the tailplane. Rather, a horizontal  tailplane test was conducted using resonance bending testing and this was carried out for  the equivalent of (in the manufactures opinion) 1000 flight hours of severe bending loading.  This test had resulted in a small crack (less than 1”) in the lightening hole, which had been  the source of the original inspection requirement, although these cracks were attributed to  gust damage in the service fleet. 

One of your fleet aircraft has just been returned to your company after being leased back to  the manufacture for their testing purposes. The manufacture had used the aircraft for  handling trials and engine running trials to refine the cooling within the engine nacelles. This 

latter testing had been carried out for 180hrs by asymmetric (single engine) ground running,  and these hours are not included in the aircraft’s total flight hours which is only 450hrs. The  tailplane was inspected visually as per the manufacturer’s instructions at 400hrs, when it is  estimated that the aircraft had been used for ground running for 150hrs. Your fleet typically  has only 5% of total time ground running and usually with both engines in operation. 

Your company has contacted the airworthiness authority, CASA, informing them of the  discovery of cracks in some of your aircraft’s lightening holes in the forward spar. CASA  contacted the manufacture, but has not received a formal response, and the cause of the  cracking remains uncertain. Your direct approaches to the manufacture have not been  responded to. The lack of response by the manufacture is believed to be the result of a re structure at the company that has seen several senior people leave. To replace the cracked  webs will be expense, although spares are available. 

You are uncertain if the cracking will lead to a safety of flight situation and the manufacture  is vague on this point, but there is little redundancy in the torque box of the horizontal  stabiliser, and if the box fails the aircraft is expected to become uncontrollable. You are also  uncertain as to the cause of the cracking and have a suspicion that the manufactures  explanation is not adequate give your diligent use of the gust locks. 

The following are questions that your team should address in your report: 

• What should your company do about the cracking? Note your company has limited  resources in both maintenance and money and any significant delay in getting  aircraft back from maintenance will cost the company dearly 

• Is your fleet safe? 

• Is the visual inspection adequate, if not why not and what changes would you  suggest? 

• If you find a small crack (<25mm) can you fly, if so should you introduce shorter  periods (how short) of inspection until a repair can be executed? 

• Can the cracked webs be repaired rather than replaced? 

• If repaired how will you assess the life of the repaired structure? 

• What short-term things can be done to improve safety? 

• Suggest a corrosion protection scheme for a repair. 

• What should you do about the returned aircraft? 

• Is the returned aircraft safe, given that it has been inspected only 50hrs ago without  the detection of any cracking in the front web? 

• The manufacture wants to re-design the forward spar, mainly to further fix the  cracking problems about the attachment points. They also intend to sell the aircraft  in the USA. 

• How will they qualify the redesign of the forward spar under the now relevant design  standards? 

• What data will they need to achieve this redesign? 

• What testing will they need to do to qualify the re-design 

• Suggest a corrosion protection scheme for the redesign.

Remember cost is a significant factor in any actions taken and may dictate the viability of  your company. 

Report preparation guidelines 

 

Introduction 

The introduction sets the tone for the report. It has two major aims: to introduce the  problem in a suitable context; to stimulate the reader’s interest. A short description of the  major findings from you group discussions and readings based on factual information and  any research your team has carried out should be provided in this section. 

Answers to the question posed 

Review the questions and try to find guidance in the literature that will help you and your  team arrive at acceptable answers to the questions. Be certain to include reference to any  guiding material that you may have relied on to reach you conclusions and also include any  assumptions that you may have used to support your position 

Analysis and discussion 

Presentation and critical analysis of findings should be included in this section.  Notwithstanding the use of other references, an effort should be made to present your  teams own arguments and critical analysis of facts (e.g. comparison with conclusions  obtained from other sources). In this section, you should aim to address the following  questions in particular: 

1) what were the key gaps and weaknesses in the manufacture’s information about the  cracking problem and what should they be doing to address this. 

2) make recommendations on what is required to address the crack problem. Conclusion and recommendations 

List any recommendations based on the discussion of the issues. An attempt should be  made to go beyond the recommendations if warranted to address the broader issues. 

Assessment Criteria 

To provide a comprehensive answer to the assessment brief, you will need to achieve the  following, as a minimum: 

• Display a comprehensive understanding of the topic;

• Demonstrate a wide review of the literature and refer to practical examples to  support your argumentation and analysis. These maybe from a minimum of two text  books/Standards and journal peer-reviewed papers from authoritative industry  sources; 

• Demonstrate a critical analysis of the literature and research, including your own  arguments based on the facts and circumstances. 

• Present information in a logical, systematic and integrated manner (i.e.,  structure/format and written quality of the report); 

• Appropriately use of RMIT Harvard Reference style. 

 

Format 

The paper is to be produced using a word-processing software (e.g., Microsoft Word) and it  should be formatted on A4 size page layout, double-spaced, in a 12-point font such as Times  New Roman, Courier or Palatino. Wherever possible, you should use appropriate graphs,  diagrams and figures to augment the text – these should be numbered consecutively and be  referred to in the text of your paper. The full detail of the source(s) of the  figure/graphs/diagrams also needs to be included in the assignment reference list. A  coversheet with the title, names, student numbers, date and word counting should be  included. 

Please write your text in good English. A high standard of presentation is expected, and  poorly presented work may have a significant impact on the final mark.

 

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