23 October 1997

Federal Aviation Administration
Aircraft Certification Office
1801 Airport Road
Wichita, Kansas 67209

ATTENTION:

Mr. Carlos Blacklock

SUBJECT:

Response to Notice of District Office Airworthiness Evaluation Items, Letter dated 08 October 1997

REFERENCE:

Final Debrief Meeting, FAA Project No. SP1729WI-A, Independence, Kansas, 03 October 1997

Dear Mr. Blacklock:

The FAA provided Cessna with a list of findings from the Special District Office Airworthiness Evaluation of the Cessna DOA CE-1. This letter is in response to those findings.

1. Cessna has always made provision to correct wing heavy conditions on the Model 172 by adjusting the aft spar eccentric for one or both wings. Hand forming of the ailerons to correct for the condition is not a process which is provided for by engineering. Cessna Engineering has coordinated with Cessna QA to assure that this procedure will not be used for anything other than secondary "fine tuning," never to be used in place of eccentric adjustment for coarse positioning. Cessna Engineering will be modified to provide for the process by modifying Drawing 0523800 with DCN 048 to permit minor adjustment of the aileron trailing edge provided that the aileron contour remains within the existing limits shown on the drawing.

2. The 172R engine installation drawing 0550359, "Engine Instl - 172" has been changed with Revision M to include specific notes which provide engine control rigging instructions for both the throttle and mixture controls. To this time, Cessna Production Flight Test has been assuring proper adjustment of the controls; however, the adjustment will first be made on the line from now on, using both engineering and manufacturing instruction changes.

3. Model 182S Engine Crankcase Breather Tube - Cessna's original design of the crankcase breather tube installation included means of preventing icing of the vent, using a hole positioned well inside the cowl with additional clearance to avoid being covered by a clamp at the cowl feed through. Cessna Propulsion Engineering verified that by using care in positioning the vent tube prior to tightening the four upper clamps which secure it in place, the original design could easily position the vent tube appropriately, providing more than enough clearance for the secondary hole. However, in order to assure that the hole would always be located as shown on the drawing and would remain unobstructed, Cessna added a minimum dimension for location of the hole above the cowl, and a note to make sure the hole remained unobstructed. The configuration shown with both versions of the drawing fully complies with subparagraphs 23.1017(b)(1) and (b)(2) of the 182S certification basis which state that, "Breather lines must be arranged so that...Condensed water vapor that might freeze and obstruct the line cannot accumulate at any point," and that it must discharge where it "will not constitute a fire hazard if foaming occurs...." However, the FAA further insisted that a change be made to the tube to add another hole, located approximately 2" below the bottom firewall clamp. A figure of the redesigned tube, P/N 0750624-17 (incorporated with Drawing 07050624 Rev. E), and its installation is attached. Cessna has included the additional hole in all Model 182S aircraft delivered after the audit by incorporation of Engineering Order EO 182-0049 (which will be effective until the production change is serialized in) or by incorporation of the new P/N tube design into the Engine Installation itself with Drawing 0750619 Rev. J. Cessna has furthermore provided a Special Service Project document, SSP97-71-01, "Engine Crankcase Overboard Breather Tube Modification," to the field which modifies and repositions the engine crankcase breather tube on each Model 182S aircraft already delivered. The release date of the SSP was 20 October 1997. Cessna will ensure compliance with the document on all 182S aircraft in the field via our Customer Service Representatives. This will ensure 100% compliance with the change, on an individual basis.

4. Model 172R Induction Drain Line - Cessna confirmed that, with care during installation, the induction drain is easily positioned per the print so that it does provide adequate positive drainage of the induction inlet elbow. However, in order to assure consistent compliance with type design, Cessna has provided specific instructions by adding a note to Drawing 0550359 Rev. M, "Engine Instl-172." An Engineering Order, EO 172-0069, has been provided to assure proper positioning of the line until the drawing change becomes effective. For those aircraft which were delivered prior to the audit, Cessna will provide mandatory Service Bulletin to inspect the drain, and reposition it if necessary. The manufacturing instruction for the 172R will be revised as well to verify positive drainage.

5. Model 172R Undrainable Fuel - The Model 172R fuel system is the same as those of previous 172 versions equipped with integral tanks, except for a reduction in fuel quantity provided by the relocation of the inboard fuel rib 7 inches farther outboard on each side, the addition of a fuel reservoir (which incorporates a quick drain), addition of a fuel pump and shutoff valve, and modification of the fuel strainer. Design and construction of the tanks themselves has not changed except to provide four additional drains per tank (for a total of 5 drains each) with all restart Model 172Rs, so that each tank will always have a drain at its lowest point, no matter what ground attitude the aircraft may be in. The FAA agreed in a meeting with Cessna on 02 October 1997 that the previous integral tank installations greatly reduced the incidence of water contamination for those aircraft so equipped.

Based upon past experience and computer modeling of the tank, the quantity of fuel required to wet previously dry inner surfaces of the fuel system components (i.e., the tank, fuel lines, gascolator, primer pump, etc.) will account for the majority of the observed undrainable quantity. A comparison of the undrainable fuel figures for the 172R and 182S would support this position, since the amount of undrainable fuel per gallon of tank capacity is very close for the two aircraft. The 172R ratio is slightly higher, as expected, due to its lower tank size in comparison to line length and fuel system components, as well as its additional reservoir.

In addition to the above considerations, Cessna removed a fuel tank access panel from the drained tank of one of the aircraft built at Independence to see where the undrainable fuel was located, and the outcome was as expected. The fuel that remained was distributed so that the bottom of the tank was wet, but no excess pooling was evident.

Cessna has enjoyed extensive satisfactory field experience with integral tanks on 172 versions already in the field. Because of this, since Cessna has changed the tanks for the 172R only in ways that will reduce the undrainable levels in the tanks, and since the tank performs completely as expected in every way, Cessna considers that this issue is satisfactorily answered.

6. Model 182S Undrainable Fuel - The issues to be considered for 182S fuel tank undrainable levels are nearly identical to those considered for the 172R. The Model 182S fuel system is the same as those of previous 182 versions equipped with integral tanks except for the addition of a fuel pump and modification of the fuel strainer. Design and construction of the tanks themselves has not changed except to provide four additional drains per tank (for a total of 5 drains each) with all restart Model 182Ss, so that each tank will always have a drain at its lowest point, no matter what ground attitude the aircraft may be in. The FAA agreed in a meeting with Cessna on 02 October 1997 that the previous integral tank installations greatly reduced the incidence of water contamination for those aircraft so equipped.

Based upon past experience and computer modeling of the tank, the quantity of fuel required to wet previously dry inner surfaces of the fuel system components (i.e., the tank, fuel lines, gascolator, primer pump, etc.) will account for the majority of the observed undrainable quantity. A comparison of the undrainable fuel figures for the 172R and 182S would support this position, since the amount of undrainable fuel per gallon of tank capacity is very close for the two aircraft. The 182S ratio is slightly lower, as expected, since the 172R has a lower tank size in comparison to line lengths and fuel system components, as well as an additional reservoir.

In addition to the above considerations, Cessna removed a fuel tank access panel from the drained tank of one of the aircraft built at Independence to see where the undrainable fuel was located, and the outcome was as expected. The fuel that remained was distributed so that the bottom of the tank was wet, but no excess pooling was evident.

Cessna has enjoyed extensive satisfactory field experience with integral tanks on 182 versions already in the field. Because of this, since Cessna has changed the tanks for the 182S only in ways that will reduce the undrainable levels in the tanks, and since the tank performs completely as expected in every way, Cessna considers that this issue is satisfactorily answered.

7. 182S Unusable Fuel Quantity - The FAA took exception to Cessna's analytical approach for determining unusable fuel quantities on the 182S, although Cessna had consistently and repeatedly shown in supporting documentation provided to the FAA that analysis was its means of compliance. Until this time, the FAA had provided no indication that the approach was disagreeable to them. Cessna's Propulsion and Flight Test specialists had evaluated the differences between the earlier carbureted engine on the 182R (according to the vendor, the carburetor bowl in the 182R holds approximately 180 ml, or 0.05 gal. maximum) and the fuel injected engine of the 182S per AC 23.959-1 para. 5.d.., and determined that since the fuel system and tank geometry remained unchanged, other differences were minor enough that previous testing would apply to the 182S as well. However, since the FAA questioned this approach during the audit, Cessna performed flight tests to validate that the analytical rationale was sound. The FAA reviewed Cessna's results and took part in certain tests specified by themselves as well, as shown by Mr. Baker's signature on the Flight Plan Card attached. The tests verified that the certified unusable fuel quantity is indeed conservative, and the results of the tests have been reviewed and concurred with by the Wichita ACO Flight Test Group and Propulsion Group. TIRDM182S-0 Addendum 2 will document the results.

In addition to the above findings, the FAA further suggested that Cessna institute a procedure whereby engineers would periodically audit Independence to monitor production. Cessna will provide for engineers to review the aircraft in accordance with the FAA's request. In addition, individual engineers who are at the Independence facility for any reason are further encouraged by management to monitor the aircraft as an ongoing DOA check.

We Trust that these answers address the given findings satisfactorily. If there are further questions or concerns, please do not hesitate to contact the undersigned.

Enclosures

L417-02-97-289

 Cessna Aircraft Company One Cessna Boulevard, P.O. Box 7704, Wichita, Kansas 67277-7704, 316941-6000