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I (think) I understand what a firewall is (at least, in a single engine aircraft where the engine is at the front of the fuselage), but what does it mean "to firewall" an aircraft's engines, as described in this incident report: is it just applying full thrust? What is the connection with the physical firewall?

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    $\begingroup$ A firewall is simpy a fire-proof wall. $\endgroup$ – David Richerby Jan 23 at 19:10
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"To firewall" is a phrase meaning to go to full power. Most aircraft throttle controls provide full power when moved to their furthest forward position - the direction towards the firewall separating the nose mounted engine from the cockpit in aircraft in the past. The phrase is still used, just as we "dial" a telephone even though the telephone dial is no longer used, either. A similar one for automobile driving is "pedal to the metal".

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    $\begingroup$ Another automotive expression I've heard is having one's "foot in the carburetor", the humorous imaging being that someone pushed the gas pedal so far their foot followed the linkage all the way to the carburetor. $\endgroup$ – Fred Larson Jan 22 at 23:10
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    $\begingroup$ Foot to the floor will do. Full bore does not get fuller. The floor being a de facto firewall where the pedal hits it in any case. $\endgroup$ – mckenzm Jan 23 at 2:03
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    $\begingroup$ Likewise, "hammer down" comes from long-haul truck drivers, when they would put the truck on sustained maximum speed while giving their foot a rest by literally laying a sledge hammer on the gas pedal. The invention of cruise control made the hammer obsolete, but the phrase remains in use. $\endgroup$ – Mason Wheeler Jan 23 at 16:01
  • $\begingroup$ @MasonWheeler that sounds like a bogus etymology to me - got a reference? Most sources indicate that it's related to the firing hammer of a pistol or rifle. $\endgroup$ – pericynthion Jan 24 at 2:29
  • $\begingroup$ I've long known it from cars, where firewall means to put your foot (on the gas) to the firewall (the wall between the passenger compartment and the engine.) $\endgroup$ – Loren Pechtel Jan 24 at 5:46
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It's just an expression. It means to push the throttle as far forward as it will go (all the way to the firewall, if you can), or full power.

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    $\begingroup$ Like AC-DC says, push the balls to the wall man. From era before engine control quadrants, ends of the controls were balls. Throttle in full, Mixture full rich, Prop control to flattest pitch for max RPM. $\endgroup$ – CrossRoads Jan 22 at 19:39
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    $\begingroup$ Streetcars typically had controllers that rotated around an axis, there's a stop molded into the top of the control stand, which was made of brass. Hence, "on the brass" indicated full power. However, you didn't want to spend any time in a resistor point, you would cruise at full series or full parallel. So "on the brass" was a normal running point, not near as dramatic as "firewalled". $\endgroup$ – Harper Jan 22 at 23:01

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