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Clearing Up Learjet (35/36) Outboard Flapper Valve Inspection Confusion

May 2010

There is a lot of confusion and frustration related to the inspection of the Outboard Flapper Valves on Learjet 35/36s. I consistently field many questions on this subject and will attempt to clarify the items that cause confusion.


Let’s take a look at these two Inspection Reference Numbers (IRNs) side by side.

  Old style only New style only (S-461 rubber)
  B4 Phase (5-10-10) Due at Other Intervals (5-10-29)
Interval 600 hours OR 2 years 24-months
AD/SB AD 95-25-03 superseded by AD 2005-16-09 SB 35/36-28-14


What is the difference between IRN M2812000 and IRN M2812003?

  • New flappers installed by SB 35/36-28-14 use an S-461 rubber.
  • There is a 5-year replacement, IRN K2812001 that applies only to the old flapper valves and goes away with the newflapper valves (S-461).
  • The B4 IRN (M2812000) only applies to old flapper valves, N/A to new.
  • The "Due at Other Intervals" (24-months) IRN (M2812003) only applies to new flapper valves, N/A to old.

If SB 35/36-28-14 is complied with per AD 95-25-03 superseded by AD 2005-16-09, which IRN applies?
If you have complied with SB 35/36-28-14, then your aircraft has the new (S-461) flapper valves. IRN M2812003 applies in this case.

AD 95-25-03 superseded by AD 2005-16-09

The 600-hour inspection in AD 95-25-03 superseded by AD 2005-16-09 was originally for all flapper valves, but subsequently an Alternative Method of Compliance (AMOC) was released on July 6, 2004, that stated the inspection of thenew (S-461) flappers was to be per the applicable maintenance manual. Then AD 2005-16-09 superseded AD 95-25-03 on September 13, 2005, in which paragraph (j)(1) basically says for aircraft with the new flapper valves installed, the repetitive inspections required by paragraphs (f) and (i) of AD 2005-16-09 could be terminated.

Although there is no specific requirement per either AD to perform a flapper valve inspection, Learjet decided to keep the 24-month inspection of the new (S-461) flapper valves in Chapter 5, "Due at Other Intervals (5-10-29)" because that is when other wing fuel inspections are performed.

Still….. Why are there two IRNs with the same calendar interval?

The question remains, since a B4 is done at 600 hrs OR every 24 months and the other IRN is performed every 24 months regardless of hours, why are there two different IRNs? The only change in actually inspecting them is the new flappers (S-461) have no life limit as opposed to the 5-year life of the     old flappers (NAS11). It would be understandable for two different IRNs if the intervals were further apart, but these are not. In my opinion, it just seems like an additional IRN that adds unnecessary confusion that is not needed.

Keep In Mind

IRN M2812000, for the old flappers, is still governed by AD 95-25-03 superseded by AD 2005-16-09 which requires a 600-hour inspection. It was placed into the B4 Phase to make it easier to track while performing the 600-hour inspection.

I hope this helps you understand Flapper Valves a little bit better and saves you frustration and confusion.