Timeline for Could commercial jets take a steeper initial glide path on landing?
Current License: CC BY-SA 4.0
14 events
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| Mar 26 at 8:54 | comment | added | mike rodent | @Mark My question is not concerned with fuel consumption. However, at this point it has been established that applying speed brakes and/or extending gear would enable a steeper descent than 3° at idle. The attempts to say that despite this there is a huge fuel consumption issue involved here ("more time at cruise" etc. etc.) are clearly bad faith straw man arguments. | |
| Mar 25 at 23:14 | comment | added | Mark | @mikerodent, ideal fuel efficiency occurs when the engines aren't running. It's inadvisable to actually shut down the engines in flight, so in practice, fuel flow is reduced to the lowest level that will keep the engines running. For an airliner, this produces a glide angle right around 3°. Other aircraft will have different ideal angles: a top-end glider will have a descent of less than one degree, while the Wright Flyer will come in at around 7°. | |
| Mar 25 at 19:04 | comment | added | mike rodent | @Bianfable Your answer still does not answer my question. Please keep politics out of this as I have been asked to do. The bolding of the phrase "nobody will do that" shows that your line of argument is fixated on following the aviation industry lines of propaganda, distortions and hypocrisy. I am asking a technical question. | |
| Mar 25 at 12:42 | comment | added | Bianfable | @mikerodent Oh yes, they sure do. Extending flaps and gear has a significant impact on L/D and therefore allows much steeper descends. Technically, you could start slowing down to final approach speed 30km from the runway, extend flaps and gear and then go for a much steeper descent (like ~4.5° to ~6.5° depending on aircraft, load, etc.), but nobody will do that due to the significant impact on fuel consumption. Also note that flying with gear and flaps extended is louder than in clean configuration, so I'm not even sure how much noise reduction this would cause. | |
| Mar 25 at 12:32 | comment | added | mike rodent | @Bianfable all very well, and I'm clearly no expert in this area: but surely drag and lift are not unvarying parameters: flaps, speed brakes, landing gear, elevator trim, maybe even loading, with passengers, goods and fuel: surely these parameters must vary the L/D settings during a given phase of flight? | |
| Mar 25 at 12:10 | comment | added | mike rodent | @Bianfable You seem to be making a claim here which stretches credulity: you appear to be saying that, for all aircraft, at all altitudes, in all atmospheric conditions, it is precisely 3° (which coincidentally just happens to be the universal ILS gradient), which corresponds to the ideal fuel efficiency. This seems very unlikely: can you provide a link where there is some scientific proof of that notion? | |
| Mar 25 at 12:09 | history | edited | Bianfable | CC BY-SA 4.0 |
incorporate comments into answer
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| Mar 25 at 11:49 | comment | added | Bianfable | @ROIMaison Sure, you could (this is just not how modern FMCs calculate T/D), but that initial shallower descent would also use more fuel (since shallower compared to an idle-thrust-descent implies using more than idle thrust). No matter how you look at it, the idle-thrust-descent gives you minimal fuel consumption and any deviation from it will come at a fuel penalty. | |
| Mar 25 at 11:09 | comment | added | ROIMaison | @Bianfable, would it be necessary to stay at cruise altitude for longer? Can't you start descent from the same point, and just use an initial shallow descent, before you proceed with the steeper descent? Perhaps that could still be more energy efficient than the stepped descent of today. | |
| Mar 25 at 10:15 | comment | added | Bianfable | @ROIMaison Because you would stay at cruise altitude for a longer time, therefore running the engines at higher thrust for a longer time, and then start a steeper descent. | |
| Mar 25 at 10:08 | comment | added | ROIMaison | "A steeper angle would require less than idle thrust (not possible) or more drag (use of speedbrakes), which increases fuel consumption and is therefore not very popular.", why would deploying the speedbrakes increase fuel consumption if the engines are kept running at idle thrust? | |
| Mar 25 at 9:43 | comment | added | Bianfable | @mikerodent At the moment, reducing fuel consumption has a much higher priority than reducing noise for residents. This is not just true for the airlines (to save money), but also for regulators and authorities (for environmental reasons). Therefore, gear and flaps will be extended as late as possible and speedbrakes should only be used when absolutely necessary. | |
| Mar 25 at 9:08 | comment | added | mike rodent | Thanks. I don't see any justification here which might lead you to make your concluding sentence. The speed brakes and landing gear could be enlisted much earlier out to help achieve a steeper path. You say that "increased fuel consumption" is "not very popular", which is not an argument. The argument about London airspace being complicated also doesn't have any proven bearing on the question under discussion. | |
| Mar 25 at 8:52 | history | answered | Bianfable | CC BY-SA 4.0 |