April 1, 2021

Announcement of
SAE AS4059F replacement of NAS1638

The NAS 1638 cleanliness classification system was originally developed in 1966 by the US Aircraft
Industries of America to both simplify reporting of particle count data and to control the introduction
of dirt during the assembly of aircraft fluid systems during the assembly of aircraft fluid systems.

The NAS 1638 cleanliness classification system is considered as out-of-date and obsolete
due to many reasons; one of those is the ACFTD (Air Cleaner Fine Test Dust) -calibration test
dust based on calibration method ISO 4402, is no longer available (discontinued).

The new calibration test dust particle standard is ISO Medium Test Dust (ISO MTD) based on calibration
method ISO11171

Comparison Particle Sizes of ACFTD and ISO MTD

SAE AS 4059 which was developed in 1988 , has undergone many revisions since then.
Now , SAE AS 4059F , the replacement for NAS 1638
Refer to SAE AS4059 rev F -2020, Table 1

Our NAS 1638 reporting will convert to or change to follow SAE AS 4059 F ,Table 1

NAS 1638 classes used in current specifications can be converted directly to SAE AS4059
contamination classes. In the simplest form, where NAS 1638 Class 6 is currently specified,

In the simplest form, where NAS 1638 Class 6 is currently specified, SAE AS4059 Class 6 applies


Yours sincerely ,
Thanant S
General Manager

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