adequately with technical uncertainties
& Quality Management
Availability, Maintainability, Safety
term "RAMS", originally "RAM" without "S", comes from defense
industry and is still today used only
in this industry, however, European railway industry has adopted the
term "RAMS" in its EN 50126 standard.
"S" in "RAMS" came decades after "RAM" has been established. However,
the "S" makes absolutely sense because the analytical methods for
demonstrating "RAM" and "S" are
- Reliability (= item will survive a mission without failure)
- Availability (= item will be in operable condition, only little
- Maintainability (= item will be easy to repair and maintain)
- Safety (= item will be safe during operation)
The following table describes RAMS in more detail.
|Item must operate with as
little failures as possible
|There are no requirements for
repair time. However, it is important that repairs occur as
rarely as possible.
|Item must be in operable
condition in as much time as possible
|Downtime must be as low as
possible. Failures are allowed, provided that repairs take only little
|Item must be repair friendly
|Item design must support quick
failure detection and short repair times. This will in turn support
|Item must enter a safe state upon
|Product may be unreliable and may
have long downtimes and poor repair procedures, provided that, if it
fails, it will enter a safe state.
the early 1960s, many standards and handbooks have been published by
the US department of defense (Mil-HDBK-xxxx
These documents contain requirements for army material suppliers. In
the meantime, almost all of these documents have been withdrawn and
made publicly available.
While de jure obsolete, many of them
are de facto still authoritative in military and even civil industries.
In contrast to standards and
procedures of many other industries at that time, these military
requirements not only addressed product quality, but also process and
The understanding that product
quality is rather the result of a good design process than an end test,
was traditionally limited to those industries with high safety
requirements (aviation, railway,
While the focus of RAMS is not
primarily safety, it was the intention of combining similar disciplines
(RAM and S) that led to the term RAMS.
Despite the differences between RAM and
Safety, the analytical methods behind are the same.
According to the author's experience,
the requirement levels can be described as follows:
methods for demonstrating RAMS requirements and goals are described here.