# Why is 5 seconds the time to respond to a TCAS RA?

With TCAS, the time to respond to an RA is 5 seconds (2.5 seconds for a reversal RA). The FCOM states that the TAU (estimated time) before the aircraft intruder reaches the "Closest Point of Approach" is 25 seconds (40 seconds for a TA). Why is it then, that pilots are required to react within 5 seconds? Is it simply to add a margin of safety (an extra 20 seconds) before an intruder aircraft comes dangerously close to collision?

If the pilot reacts at impact it is too late to avoid collision

If the pilot reacts one second before impact it is too late to make any significant altitude change and to avoid collision.

If the pilot reacts two... you get it.

The TCAS algorithm calculates with manoeuvres of approximately 20 seconds to create sufficient vertical spacing to avoid a collision. An extra 5 seconds margin is added for the pilot to initiate the manoeuvre after the RA is given.

So there are 5 seconds for safety margin, and 20 seconds for the laws of physics.

• I believe that 20 seconds is also so people in the aircraft don't die from the maneuver. So an otherwise-equivalent flight with no passengers could potentially have a smaller window. An A320 is about 40 feet tall from the bottom of the engine to the top of the vertical stab. Normal climb is 2000 ft/min, or 33 ft/s. Rapid ascent/descent can be much faster. With both pilots reacting, it's twice as fast. I.e., it doesn't take close to 20 seconds for the aircraft itself to get out of danger altitude. Mar 18, 2022 at 14:47
• There was a Japanese flight some years ago where the pilot reacted at the last seconds (estimated about 2 seconds before impact), narrowly avoiding a collision, but multiple passengers and flight attendants were injured as a result. en.wikipedia.org/wiki/2001_Japan_Airlines_mid-air_incident (It was a 747 there, but I don't think the performance is that different for my point.) Mar 18, 2022 at 14:52
• Why not more than 5, or more than 25 s total? I'm guessing because false positives are more likely if you have to predict a collision too far in advance, and you want TCAS to be treated with urgency, not a common occurrence. Mar 18, 2022 at 15:01
• When you get an TCAS climb or decent, theres very few pilots that will spend time looking for the reason. Its a bit like the fuel gauge; I dont believe if it says full, but I do if it says empty. Just react, get out of the way, then sort out the reason. Its why acting in accordance with an RA is legal without telling ATC (until afterwards) if theres not time, even in controlled airspace. Mar 18, 2022 at 15:09
• @PeterCordes, it's due to false positives. At typical cruising altitudes and speeds, minimum lateral separation is about 38 seconds.
– Mark
Mar 19, 2022 at 0:45