Hawker 800XP 103278-13S1 vs 103278-13S2 vs 103278-18

The short shop answer

Keep all three numbers separate until the airplane configuration is known.

The IPC gives enough information to show that the records are related by installation subject, but it does not give a one-line statement that S1 always becomes S2, or that every S1 and S2 unit becomes -18.

Part number	Main printed configuration language	Rows	Quantity	CMM
103278-13S1	Pre Mod 25F586C, pre SB21-160, and pre SB21-3621 depending on row	6	1 per row	21-30-56
103278-13S2	Kit 4200097-3, Mod 25F586C, SB21-160, and pre SB21-3621 depending on row	6	1 per row	21-30-56
103278-18	SB21-3621	2	1 per row	21-30-56

That is the useful part of the catalog answer. The aircraft records and current approved data finish the eligibility check.

What the S1 rows actually say

103278-13S1 appears in Figures 21-30-05-10 and 21-30-05-10B at three separate item positions.

Four rows call it pre Mod 25F586C and pre SB21-160. Two rows call it pre SB21-3621. Those two descriptions are not interchangeable notes. They belong to specific serial blocks and item stacks.

If the removed unit is S1, put these items on the request:

• complete part number, including S1
• aircraft serial
• figure and item when known
• installed position
• Mod 25F586C status
• SB21-160 and SB21-3621 status
• unit serial and data-plate details

What changes on the S2 record

Four 103278-13S2 rows reference kit 4200097-3, Mod 25F586C, and SB21-160. Two other S2 rows are marked pre SB21-3621.

That mix is why the suffix cannot be used as the only effectivity check. S2 identifies a different catalog record, but the source still requires the mechanic or buyer to carry the exact row context forward.

Commercial cross-reference pages commonly place S2 beside other 103278 numbers. Use that as a way to find inventory or a repair source. Do not use it as the technical approval for substitution.

Where 103278-18 fits

The -18 number appears twice:

• Figure 21-30-05-10, item 15
• Figure 21-30-05-10B, item 90

Both rows carry SB21-3621 language, quantity 1, and CMM 21-30-56. The serial blocks divide between the two figures.

The catalog still does not print a universal statement that -18 replaces every S1 or S2 valve. The safe lookup is to verify the completed bulletin and modification configuration, then check the current approved effectivity.

Do not total the six-row quantities

S1 and S2 each have six application rows. That does not mean six valves are installed.

Every row prints quantity 1 for its applicable item and configuration. Multiple rows exist because the IPC divides the installation by figure, item, modification state, service-bulletin state, and aircraft serial.

What a useful quote should show

A useful quote identifies the offered part number exactly and states its condition, traceability, release, warranty, and any core requirement.

For this family, it should also carry the airplane serial, installed position, MOD status, service-bulletin status, and whether the quoted source is claiming a direct exact-number unit or an alternate. Any alternate needs its own approved compatibility check.

What this guide does not establish

This page does not provide valve test limits, removal or installation instructions, pressure schedules, rigging, leak checks, or return-to-service criteria. It also does not substitute commercial cross-reference language for approved technical data.

Use the current applicable maintenance manual, CMM, service bulletin, and aircraft records for those decisions.