Originally Posted by
ItsonlyMeagain
You are correct that a 3 degree flight path with a fixed aiming point is fixed in space; I did not dispute that. However to maintain that with any wind, the variable is power. In a strong tailwind there will come a point where this is not possible as the power will be at idle. Of course, the way to maintain the flight path now is to point at the aiming point as the tailwind tries to put you above the profile. Should one arrive at the aiming point at the correct height, you will certainly be fast and….. you will land long. Professional pilots call this an unstable approach.
Me
Me; the effects of inertia means that IAS is affected unless the "shear" (change/time) changes are very slow rate, and/or the aircraft has low inertia. Within an airmass, the aircraft is moving but takes time to accelerate the inertial mass, so the GS does not instantaneously increase by 50kts in a sudden 50kt reducing headwind, instead, the IAS (CAS) alters immediately, it is not subject to inertia, and the ground speed alters rapidly but not instantaneously until reaching equilibrium (turning a B747 to the NE over Kushimoto with a 250kt jet stream, where the component would go from a 100kt head to about 250 tail was always worth a bet for the first beer in Narita, a 350kt shear in 45 seconds was always worth watching). Slow rates, no problem light planes, no problem.
Re "power is the variable", that is not the issue with most reverse shear cases. The attitude has to change as the first effect of the drop off of a tailwind is an increase in IAS, that causes a pitch up if open loop control due to speed stability. That acts to reduce sink rate, while at the same time, the rapidly increasing ground speed now necessitates an increase in sink rate, so the immediate effect open loop is to go above glide slope. Closed loop, the pilot has to add a nose down input to avoid the balloon from the additional IAS increment, which also needs further adjustment due to the increased sink rate requirement over time as GS increases to equilibrium condition. The increased speed requires a power reduction, (first power change, more to come). As the conditions stabilise, the IAS starts to reduce other than the increment that occurs from the lower attitude required for the higher sink rate, which then requires an addition of thrust to stabilise back at the equilibrium condition, which hopefully is back at Vref + additive, and with the power slightly lower than prior to shear entry, and with a slightly lower attitude to maintain path. The problems arise when the crew carry excess speed into a reverse shear, and cannot get the aircraft to return in time to stable conditions in order to be stable into the flare. Stable here is still within the generic requirements for a stable approach that companies will specify in their OM's.... The routine problem from the investigators view is to determine whether the crews actions were within normal boundaries for the conditions. Carrying excess speed into a reverse shear is a headline that the crew are not cognisant of the dynamics that will occur (the opposite is true for a normal shear event). A ballooning out of the slot during transition of a reverse shear is indicative of the crew not adjusting the attitude for the speed change, excess IAS correlates to inadequate thrust reduction during the transition; low IAS/energy into the flare is indicative of failure to restore thrust before the flare... A factor that often shows up on hard landing investigation is that the crew take the power off due to the excess IAS, and then do not add it back to somewhere near a normal thrust level before entry into the flare...
All of this takes more time to say than do.... but the sequences of actions are constants, and show deviations from desired controlled state.
Tin hat time.... the B767 that was climbing at idle as it entered a spectacular shear from the jet wind... climbing at MMO with speed brake out and thrust at idle. Impressive.
Or the B737-800W that missed its level off by 4000' going up, with a 1.72g pull up, a 0.2g over the top, and the subsequent 1.7g pull to level off at the assigned altitude. All at idle and with the speed brake out, at MMO. Neat jet stream.
All of the above barely rises above the point of boredom, but if the driver starts briefing about adding speed for mum and the kids for an expected reverse shear, then it is apparent they don't comprehend what the aircraft does in shears. We get the unfortunate lot to be presented with interesting conditions that appreciate preparation, and occasionally a rational "er, no thanks, I'll take the other runway...". As a group, we routinely try harder than we should to accomodate lunacy arising from bureaucracy, and the outcome occasionally is a desire to have a repeat of the last 10 seconds again while sitting in the wreckage.
This may seem like teaching to suck eggs, but the number of pretty shambolic approaches that get analysed suggests that we can occasionally do with clearing our thoughts. For the non flying driver, it is even more important that he or she understands what is going to happen next. The side kick doesn't have the input of the control feel to include in the process so can only rely on the performance gauges and comprehension of "plain fiziks" to be inside of the loop in a meaningful manner. In the really ugly events that come across the desk, the data shows frequently around 4-5 seconds of prior deterioration of flight kinematics before the day gets mussed up. (slightly less I guess for the Mega Deaf II, if I recall correctly).
The wise words of Mme 'Bussé and Mr Boing for all of their shiny toys invariably indicate that thrust is one of the factors in determining whether stable or not, and almost every nasty landing that I have had to analyse had the thrust well away from a normal fist position crossing the threshold. Being at idle already or TOGA thrust at that point may result in expenses. (results may vary...
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