Inspection Frequency Tolerances: When am I out of compliance?

Nov. 1, 2003

Inspection Frequency Tolerances
When am I out of compliance?

By Joe Hertzler

Scheduled inspections on aircraft include, in most cases, an inspection tolerance allowing the operator to fit the scheduled maintenance in around other demands on the operation of the aircraft. These tolerances are an integral part of the inspection program the aircraft is subject to. The FAA has not created a standard inspection tolerance matrix that covers all aircraft. The creator of the inspection program that aircraft is maintained to is also the creator of the inspection tolerance.

This month we will look at the different types of inspection tolerances and show some examples of how those tolerances affect the inspection schedule for the aircraft. For the purposes of this article let's consider multiturbine powered aircraft that are subject to an inspection program defined by 14 CFR Part 91.409 f. Inspection tolerances also affect air carrier aircraft under Part 135 and Part 121 but the inspection tolerances specific to an air carrier operator are defined by that air carrier's operation specification.

Maintenance on an aircraft is made up of several different things. The FAA's definition found in 14 CFR Part 1 states, "Maintenance means inspection, overhaul, repair, preservation, and the replacement of parts, but excludes preventive maintenance." An inspection program lists several maintenance items and ultimately makes up the maintenance schedule. There are look-phase inspections, special inspections, overhaul or replacement items, lubrication and servicing items, and airworthiness limitation time life items.

In addition to laying out the schedule of events, the inspection program will usually include a section for inspection tolerances. Inspection tolerances don't generally apply to scheduled part replacement or overhauls but rather to look-phase inspections and special inspections.

Let's take a look at the different types of inspection tolerances along with examples of how they are used.

Tolerance window
The most common inspection tolerance is probably the tolerance window. The concept here is that as long as the inspection item is accomplished within the allowed window of time, the next due time for the same inspection item does not change. For example, the Learjet inspection program for the Model 35 includes the following description:

Ref MM-99 Section 5-10-00 Page 7
1. Inspections controlled by calendar time may be accomplished within a period beginning two weeks before and ending no later than two weeks after the inspection due date.
2. Inspections controlled by flight hours may be accomplished within a period beginning 25 flight hours before and ending no later than 25 flight hours after the inspection due date.
3. Inspections controlled by landings or cycles may be accomplished within a period beginning 25 landings or cycles before and ending no later than 25 landings or cycles after the inspection due date . . .
5. After completion of an inspection, the next due time shall be at the scheduled time, date, or cycle as calculated from the due time of the last completed inspection, not from the point of completion.

In this case, if the inspection to be accomplished is next due at 1,200 hours, it can be accomplished at hours including and between 1,175 hours and 1,225 hours and the next due time for the inspection will remain as scheduled at 2,400 hours. If, however, the inspection is accomplished earlier than the beginning point of the window, say at 1,150 hours, the next due time will be 1,200 hours following completion or 2,350 hours.

Similarly, if the inspection is accomplished later than 1,225, the ending point of the window, the operator has exceeded the allowable term for the required inspection and is out of compliance with the regulation. The next due would then be from the completion time of the inspection rather than the due time.

Overflight allowance
The overflight allowance inspection tolerance is much less common and is generally associated with engine inspection for the purpose of allowing the operator to fit the required engine inspections specified by the engine manufacturer in with the airframe inspections specified by the aircraft manufacturer. An example for the overflight allowance tolerance comes from the Honeywell TFE731-5-1H engine inspection program. See the following excerpt:
Ref. Honeywell Light Maintenance Manual TFE731-5/-5AR/-5R (Report No. 72-02-75); Section 72-00-00 Page 619
Inspection is recommended at intervals of engine operating hours specified in Table 603. Specified inspection intervals should not be exceeded by more than 10 hours of engine operation.

Here Honeywell simply tells us that the interval should not be exceeded by more than 10 hours. So, in the case of the 300-hour inspection for example, we have until 310 to accomplish the inspection. Regardless of when the 300-hour inspection is accomplished, the next due time for the inspection is then calculated from the completion time rather than the time it was due. If we complete the 300-hour inspection at 306 hours, the next 300-hour inspection is due at 606 hours. If we complete the 300-hour inspection at 276 hours the next 300-hour inspection is then due at 576 hours.

The end-of-month inspection tolerance has its roots in the requirements for annual inspections required for piston-powered small aircraft and rotorcraft. The regulation - 14 CFR Part 91.409 (a) states, "Except as provided in paragraph (c) of this section, no person may operate an aircraft unless, within the preceding 12 calendar months, it has had - (1) An annual inspection . . ."

The key phrase here is "calendar months." Although there is not a specific definition of "calendar month" in the regulations there are several references throughout the advisory circulars with examples of the intent of the term. What "calendar month" means is that if an inspection is due to be accomplished every X number of calendar months, the next due date is the last day of the month that the inspection drops due. For example, if an inspection due every 36 calendar months was last accomplished 8/14/2001 the next due date for that inspection is 8/31/2004. A common practice for operators using frequencies in calendar months is leap-frogging. This is the practice of taking the aircraft in for the inspection due on the last day of the month and then picking the aircraft up in the next calendar month. The result is basically an extra month each time. Leap-frogging is not illegal, however it is the reason that in the view of the FAA there is a clear distinction between "calendar months" and just "months." Basically, if the inspection program does not specifically call out the inspections due in calendar months the calendar limited inspection is due on the day it was accomplished X number of days, months, or years later as the case may be. Aircraft manufacturers generally do not call for end-of-month tolerances. These tolerances apply basically to those requirements that come directly from the regulations. For example, 14 CFR Part 91.411 reads as follows:

"No person may operate an airplane, or helicopter, in controlled airspace under IFR unless - (1) within the preceding 24 calendar months, each static pressure system, each altimeter instrument, and each
. . . have been tested and found to comply . . ."

This language is used in several regulations and is universally interpreted by the FAA to mean end-of-month for those inspection requirements.

Hard time
If the inspection program that is currently being used for your aircraft or the aircraft that you are managing or maintaining does not specifically call out an inspection tolerance then the inspection time limitation is a hard time. This applies generally to all airworthiness limitations and overhauls and replacements. The hard time simply means that the inspection or time change must be accomplished no later than the scheduled due time. To fly past that time would constitute a violation of the regulation.

Now let me share a couple of examples of the impact of inspection tolerances and the importance of understanding their implication. Recently Cessna discovered that several airworthiness limitations were sprinkled in and among the phase inspection program for the Citation 560. Because these items were included in the basic phase inspections, the inspection tolerance allowed by Cessna for the inspection also were being applied to the airworthiness limitations. The result of the discovery and subsequent corrective action was the creation of the "M" Phases - Phase MA, Phase MC, Phase MD, Phase ME, Phase MF, Phase MG, and Phase MH. The description for each new M Phase inspection includes the following statement, "This inspection is a Chapter 4 requirement; therefore the interval limitation cannot be exceeded." I draw out this example simply to emphasize the importance of a complete understanding of the inspection program your aircraft is subject to. You cannot apply the inspection tolerances allowable for the other phases in the program to the M Phases. In some cases this causes a maintenance-scheduling problem.

Another example is the Raytheon King Air 200 Series inspection program. Under inspection interval tolerance in the maintenance manual they call out different tolerance for the phase inspections than are called for under special inspections. For the phase inspections there is a tolerance window of +/- 20 hours. For the special inspections it allows +/- 12 days per 12 calendar months not to exceed 60 calendar months and then +/- 10 percent for cycle and hourly limited special inspections. Next, just to complicate things a little, Raytheon requires that all four phases be completed at least one time every 24 "calendar months" putting the deadline for the completion of all four phases at the end of the month rather than the +/- 12 days tolerance for special inspections. Confusing??

Each inspection program, whether written by the manufacturer of the aircraft/engine or the air carrier certificate holder should be designed to ensure the safest operation possible and provide for the completion of the inspection in the most efficient way possible. The inclusion of inspection tolerances provides for the flexibility needed to schedule the maintenance in an efficient manner. If you haven't spent the time to really understand the inspection program and its inspection tolerances associated with the maintenance program you are subject to, it might be worthwhile. You may find that you have more flexibility than you thought.

Joe Hertzler is the president of AVTRAK Inc., an Aurora, Colorado-based company. Joe is an Airframe and Powerplant mechanic with Inspection Authorization and also a private pilot.