Linked directly to the nervous system, an automail prosthesis does not usually require an external energy source, and can be moved at will like a natural appendage, taking electrical pulses from the nerves to power and regulate the various electric motors and pneumatic actuators inside. Automail limbs are generally made with a full steel skeletal frame layered with steel armor-plating to protect the various wires and intricate machinery inside; however, contemporary technological advancements have allowed certain automails to be comprised of other materials such as carbon fiber, fiberglass, chrome, copper-nickel and aluminum alloys, so as to be less cumbersome and more durable than steel in abnormal conditions.
Though arguably useful, the set of drawbacks unique to automotive prostheses is enough that many are loath to undergo the upgrade from standard prosthetic attachments. The installation process, which requires a specialist in bio-mechanical engineering (that is to say, both advanced mechanical engineering and surgical medicine), is understandably expensive and notably painful, as certain body parts must often be further amputated in order to accommodate new automails properly and the sensitive nervous system registers all types of over-stimulation involved as intense pain. Those receiving automail for the first time must also have each nerve painfully connected to a permanent housing system on the body, as well as have all the nerves connected simultaneously when the limb is later attached to the housing system. The recovery from surgery, including the period of time necessary to fully master the use of automail limbs, is noted to take roughly three years on average (though full recoveries lasting as short as one year have been documented, such as Edward Elric's, and a recovery period as short as six months was stated as a possibility but was not confirmed) and even after such time, automail prostheses must be meticulously maintained with regular oiling and frequent overhauls which necessitate that the automotive parts be disconnected from and then painfully reconnected to the nervous system in order to remain in proper working order.
Like wearers of normal prostheses, automail users are subject to various aches and pains resulting from drastic changes in barometric pressure, however another drawback to full steel limbs are the adverse effects which occur in extreme conditions such as heat and cold. In extremely high temperatures, the metals - which conduct and retain heat, induce great deals of discomfort to the user, who must remain with a heated metal object grafted to the skin; and in extremely cold temperatures, the cold conducting metals cause standard oils to cease functioning as well as expedite frostbite damage to the connecting tissue.
A lesser documented setback, mentioned by engineer Dominic LeCoulte, is that automails put a large strain on the users' physical development which impedes growth.
CustomizationThough automails are generally used as an advanced medical tool to facilitate the lives of amputees, within the country of Amestris (which has seen a significant rise in automail usage in its past two decades of constant, successive wars), automail has developed into a venue for weapons development as well, resulting in "combat automail". Using various techniques, these machines, previously used only for movement, have begun to incorporate armaments such as diamond-tipped claws, hidden blades and even firearms, sometimes willingly at the expense of normal limb functionality. As such, there has also been a rise in the number of fanatical automail hobbyists who willingly sacrifice flesh-and-blood body parts by way of operation in order to obtain weaponized combat automail appendages.
While implausible, it also appears that such things as "full-body" automails exist, taking the place of vital organs and sometimes comprising half or more of the body's shape and/or normal functions. However, this is only shown in the 2003 anime and alluded to in the manga.
- Edward Elric - The protagonist of the series, Edward dons both an automail right arm and left leg, built by his childhood friend and engineering prodigy, Winry Rockbell. After his first model (a full steel automail) is destroyed, Winry builds him a new model with a higher percentage of chrome (17% of the overall makeup) incorporated to make the steel more rust-resistant. When traveling in the snowy north, Edward switches once again to lightweight, cold-climate versions of both his arm and leg automails tempered with aluminum and a high percentage of carbon fiber. In the Fullmetal Alchemist movie, Conqueror of Shamballa, Edward wears a special gas-powered automail arm built by Hohenheim while living in the alternate dimension. At the end of the manga and Brotherhood, Edward has regained his original right arm, but still retains his automail leg.
- Bald - A high-ranking member of the Blue Squad terrorist organization, Bald's automail left arm houses a high-caliber, double-barreled rifle in exchange for normal functions and also contains a hidden army knife.
- Den - The Rockbell family dog is also an automail user, having had her front left leg replaced with an automotive prosthesis at some point before the start of the series.
- Paninya - After a terrible train accident, young Paninya had both her missing legs replaced with automails built and maintained by Dominic Lecoulte. In addition to being remarkably lightweight, each leg is also fitted with an internalized weapon - with the right shin housing a hidden blade and the left leg concealing a 1.5 inch cannon. In the 2003 anime, Paninya also has an automail right arm.
- Tetsu - A young boy living in Rush Valley, Tetsu relies on his automail leg, built and maintained by Winry Rockbell, for support.
- Captain Buccaneer - Buccaneer, an officer stationed at Fort Briggs has been shown to use two types of combat automail in addition to his rarely-seen standard model, swapping one out of his right shoulder's socket for another based on the combat situation he plans to enter. His initial combat model is the M1913-A "Crocodile" which, like Bald's arm, abandons normal arm functions in favor of a weapon - in this case a gas-powered chainsaw with a movable jaw to trap objects. This particular model combats frostbite by warming Buccaneer's shoulder with the exhaust of the gas engine. Buccaneer later switches temporarily to the M1910 "Mad Bear Grade", a large and durable standard arm adorned with diamond-tipped claws at the end of each finger.
- Lan Fan - After losing her left arm on the battlefield, Lan Fan had a combat automail replacement built for her while abroad. It is adorned with a spike strip wound around the hand as well as a long sword blade extending from the bottom of the elbow.
- Frank Archer - Exclusive to the 2003 anime series, Lt. Col. Frank Archer underwent an unlikely automail installation after nearly half of his body had been destroyed during the disaster at Reole near the end of the series. As a result, the damaged part, including most of his head, was replaced with heavy combat automail. His left arm became a foldable anti-tank rifle while his mouth concealed a mini-gun. It appears that an analog vocoder has also been implanted into his system, as he is capable of speech despite the mini-gun. This type of Automail should not be considered canon with the manga series and Brotherhood, however.
- Wrath (2003 anime) - After losing his human right arm and left leg at the end of the 2003 anime series, the Homunculus Wrath is fitted with automail replacements similar to those of Edward Elric.
- Genz Bresslau - A character exclusive to the video game Fullmetal Alchemist and the Broken Angel, Colonel Genz Bresslau is fitted with a large automail left arm decorated with camouflage paint.