pmholling.bsky.social
Senior Lecturer in Aerospace Engineering, expert on uncertain decision making- Retweets are things I find interesting, but do not, always reflect my opinions.
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Like many things, that made more sense in the post financial crisis, pre-pandemic world. The pandemic put paid to the ‘old’ world of bottom limit.
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The only times I have seen it happen, outside of during turbulence, is when items were place on top of each other in the older style lockers. They would end up resting against the door. Obviously it has happened at least once.
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Small is relative, but yes they can be substantial areas. They are open to the public as a condition if the properties being built.
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This is quite common, especially for play areas and small green spaces. The councils demand this as part of planning. There are mandated to be open to the public, but a private provision. Allows the councils to meet recreation targets without a burden on their coffers.
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Of course all of this will be revealed if/when the FDR and CVR are recovered and analysed. Airspeed, angles, accelerations, engine conditions, etc, plus control surface and flap settings will all be analysed. So too much speculation as to the sequence of events is un-warranted and ill-advised
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but the deterioration would be much more gradual and the rate of descent not all that much higher, at least initially. Keep in mind the entire #AI171 flight lasted about 1 minute.
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However, you could end up with an aircraft that only pitches down a small amount, but descends quite slowly into the ground. If you maintain pitch you would eventually enter the 'stall' regime,
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Keep in mind the 787 has soft envelope protection, so stalling the aircraft in normal law isn't trivial (and would show up on the CVR)
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If, however, you have a loss of thrust, and the quite typical human response you would see an aircraft maintaining pitch (especially at the start as it is likely the pilots where 'flying pitch') but that the rate of climb, forward speed and AoA would deteriorate. Again the specifics are not simple.
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To know what the exact trajectory would look like would require dynamic simulation, but it wouldn't be that subtle. Keeping in mind that in commercial operations we try and keep the accelerations to around 1.15-1.2g. Of course you could pitch up more subtly.
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If you pitched up to stall at that point you wouldn't have a much higher pitch angle (25°-30°), but you would get quite an rapid increase in vertical acceleration as you bled off speed (initial would be about 3.7 m/s^2 or about 1.4g). That is quite a change and would be noticeable to anyone watching
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Keep in mind a normal flight at 175 knots airspeed and 3000 fpm climb would have a pitch angle of 20°-25°. That is quite an angle to an outside observer.
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likely FLAPS 1 or 5 as you can see the slats, but not much trailing edge deflection. The flight would have had to have a tailwind of 20+ knots to be approaching stall. The wind at the time, on the ground was nearly an 8 knot tailwind. That would be a lot of wind shear, 28 knots in less than 100 m.
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The last reported position on the normal @flightradar24.com feed indicated a ground speed of 174 knots and barometric altitude of 174 knots. Given the flaps configuration you can see from the phone video posted online,
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It would also be quite noticeable, as the angle of attack would approach 75° unstalled and even higher if stalled. Given from the videos available the pitch (angle of the aircraft to the horizon) looks to be about 30°-45°. That would mean the flight path would be downward between 30° and 45°.
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Since we are talking excess thrust a loss of ~75% of thrust would get you there, or you could slow down to about 40 m/s (77 knots), provided you didn't stall the wing. Of course that is well above C_L,max for a 787 even at max flaps.
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That isn't particularly high for a commercial transport, but would be about right for just after take-off with the gear hanging down and appropriate flaps on a hot, ~37°C, day. For a 787-8 that would be about right for a flight from AMD-LGW. So what does it take to get that RoC to 0.
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So back to trajectories. Imagine I have an aircraft with a a thrust to weight (T/W) ratio of 0.25, that is climbing at 16.67 m/s (~3,000 ft/min), while flying at 90 m/s (~ 175 knots). This means that the L/D is ~15.
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So a heavier aircraft will, all else being equal, have more induced drag than a lighter one when flying at the same speed. This means the heavier it the aircraft is the faster things go wrong.
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Since we define C_L in proportion to the Lift over airspeed squared, and for unaccelerated flight lift is basically equal to weight and aircraft that is 10% heavier requires 10% more lift, and either 10% higher C_L and AoA, or ~5% faster airspeed.
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Depending on how heavy an aircraft is at the time the angle of attack will be different at the same speed. C_L in the normal flight regime is basically linearly proportional to angle of attack. to increase C_L by 12% you need to increase AoA by 12% (AoA is also commonly referred to as 𝛼)
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That will depend on how heavy the aircraft is at the time. This is because the point of minimum drag is determined not directly by speed but be the angle which the aircraft 'approaches' the air it is flying through. This is called 'angle of attack'.
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This is referred to as 'lift induced' or just 'induced' drag. The general rule of thumb is that induced drag is proportional to the square of the lift coefficient. So that 12% increase in C_L gives a 26% increase in C_D,i. This doesn't necessarily mean the total drag increases by 26%.
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It is well below what is known as the 'minimum drag airspeed'. This means that in this regime the drag of the aircraft is 'dominated' not by the forces of the air flowing around the body, but by the 'force' required to turn that air and generate lift.
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this is called the lift-coefficient (C_L), by 1/square of the velocity ratio. So for an aircraft that slows from 180 knots to 170 knots, the C_L has to increase by 12%. That doesn't sound like a lot. However, for a commercial aircraft 180 knots indicated is relatively slow.
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What happens is that as you slow down the drag increases. In the 'normal' flight envelope it increases in a pretty nasty way. For a given amount of lift (the force). You need to increase the amount the wing 'turns' the air,
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So a complete loss of lift would lead an aircraft accelerating downwards at somewhere between 0g and roughly 0.25g (1g is no acceleration downwards – you have to love reference frames). Of course aircraft don't completely lose lift when flying, especially not if they are intact.
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To determine that you would need to look at the accelerations. Commercial transports have a thrust to weight ratio, at maximum take-off weight, in the 0.23-0.33 range (it can be higher in some situations).
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So when people tell you the 'aircraft lost lift'. The first thing to look for is does the flight path (the trajectory the centre of mass of the aircraft takes) change rapidly. If it isn't then it likely isn't a loss of lift situation. Even if it does, it doesn't mean that there was a loss of lift.
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Of course when you transition from one state to another, things get a lot more complex. So if you see an aircraft climbing or descending in a steady manner that tells you there is a 'surplus' or 'deficit' of thrust. In this case lift has little to do with what you are seeing.
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If you have 'excess' lift you don't climb steadily you 'turn' in the vertical plane, that is you pull up. Keep it going and you will end up inverted. If you do this in the right place with the right aircraft it is really fun.
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which is a typical value for a commercial jet is only about 9° low altitude speeds, and lower at higher airspeeds. Doesn't require the aircraft to generate much more lift than level flight (9° is about 1.2% more lift).
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This is important because unless you start from this you will get things wrong all the time. In the steady state aircraft climb gradually in the atmosphere because they have more thrust than is required to balance drag. Even a Rate of climb of 3,000 ft/min (~30 knots, 15 m/s),
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One of the areas that a lot of people struggle with is climb performance (also glide and descent performance). Aircraft fly through the atmosphere and outside of roughly 1/2 to 1 wingspan* away from the ground the ground is basically irrelevant.
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I spend a lot of time trying to teach students about the physics and they get it wrong on a consistent basis. It takes quite a long time to wrap one's brain around everything and you can quite easily end up in places that 'feel' right but are completely wrong.
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However, I have noticed there is a lot of commentary from 'experts' who don't seem to understand the basic physics of fixed-wing aircraft flight. Some of these folks are pilots, others are 'aviation safety experts'. The issue is that the physics are not really intuitive.
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The pilot who doesn't want to go down pitches up more, which further increases the RoD.
In a loss of thrust event you want the least amount of flaps possible (see the pilot's response on BA38), though having the gear down a bigger killer of RoC than Flaps 20 vs 5
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pitch up. Likely from a combo of loss of airspeed and pilot reaction. The first bit is a dynamic response, the second is a human response, especially at low altitudes. This then gets you 'behind the power/thrust curve'. Where the drag increases and the RoD increases.
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The trajectory in the CCTV footage, especially when married to the earlier phone footage is a classic 'loss of thrust' trajectory. The ground roll, rotation, and 1st segment of climb don't look anomalous in any way, but then the aircraft starts to lose vertical speed, followed by the 'natural'
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You would need to watch other departures in similar conditions to have know if it is 'normal'.
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Yes, the ACAP indicates that, I seem to remember, Flaps 20 is the 'default' for the altitude and temp. However, if the aircraft was lighter than max for the conditions, or if there are obstacles on 2nd segment you might want to use a 'lower' configuration. So Flaps 5 might be 'ok'.
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The flaps were at least at ‘Flaps 1’ and could easily be at ‘Flaps 5’. Trailing edge deflection for the latter is quite small. The slats are clearly visible in the earlier video. Whether Flaps 5 would be a misconfiguration I don’t know.
That said max climb rates are higher with less flaps.
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Prior flights at that point in their trajectory had ground speeds of ~185knots. As you say without the wind information it is hard to make any judgement
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Forgetting the flaps would not, on its own create this trajectory. A 'smooth' glide indicates a lack of 'sufficient thrust'. This would likely require issues with both engines.
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Except it doesn’t really. At least at no where near the rate it used to.
In the last 12 months, worldwide there have been 17 fatal and or hull loss accidents for commercial aircraft. 4 have been on Boeing aircraft, 3 on ex-Bombadier, 2 on Airbus aircraft.
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N
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I’m guessing it’s the Beaver on the right.