Lemaurin Re “#338. The autobrake use does not prevent you from judging by yourself, either visually or through the seat of your pant (ies) whether your deceleration is adequate or not, so your comments on this respect are a bit strange to me.”
As stated, auto brake does not ‘prevent’ judgement of deceleration, we agree; but I suggest that it increases the difficulty / complexity of the judgement.
- With pure manual brakes pilots form a relationship (experience) between the tactile input (foot force/position) and the deceleration achieved. Variations in this can be interpreted as a change in runway friction (ignoring brake failures which are normally alerted / indicated independently).
- Using reversers has a similar tactile / deceleration relationship, although with greater variability with speed, but alone does it does not relate to runway friction, i.e. less info.
With brakes and reverse, the result is similar to brakes alone, a complete, meaningful feedback loop. With experience, a pilot might judge the component of deceleration contributed from each system.
- When auto brake alone is used, pilots are distanced from the (deceleration) system by the reduced correlation between the input and the sensed deceleration (no tactile feedback from the DECEL light), and auto brake lacks any modulating control – no variable foot force, thus an incomplete loop.
- The combination of auto brake and reverse further distances the pilots from the system as it is more difficult to judge what proportion of the deceleration is being provided by each system. This is adequately explained in the Boeing presentation 'Flt Ops / Flying Technique … Landing on Slippery Runways'. (from #307) where safetypee outlines the problem of perception and the likelihood of misinterpreting poor braking on a low friction runway as a brake failure when reverse is cancelled.
Overall, the use of auto brake during landing adds ‘complexity’ to the operation with little benefit. Those advantages quoted are pretty ‘soft’ excuses for having auto brake during landing in comparison with the originating need during RTOs – failure to apply max brake.
Finally remember that as an older pilot, presumable with greater experience, the viewpoint of auto systems is not the same as an less experience pilot, who in modern operation probably has less opportunity to gain the high levels of experience to perceive/deduce adequate aircraft/system interaction, e.g. lower frequency of manual brake landings.
The skills issues here are those of perception and system/aircraft performance relationships; the difference between knowledge, as in ‘know-what’ and knowledge, of ‘know-how’.