# Could commercial jets take a steeper initial glide path on landing?

The context here is London. The aural environment of 9 million people (or so) is impaired significantly by the fact that we have planes landing at LHR all day long. LHR was established on the wrong side of London, in the sense that the prevailing winds almost always blow from the West, but LHR is to the West of London. So 90% or so of landing planes overfly the inhabited city.

A 3 degree slope on final may be something which can't be avoided for all I know, but might it be feasible to have, say, a 5 degree slope until the planes were a few nm/km from the touchdown point?

As a Londoner who's had no choice but to put up with this for years/decades, I have noticed that an A380's height overhead makes an immense difference to the reality of the noise nuisance and dB level experienced.

I live something like 20 km from the touchdown point (planes take a curving trajectory, before lining up more or less with Thames to the east of LHR runways), meaning that when I do the maths (the sine of 5° compared to the sine of 3°) I have no doubt this could make a lot of difference to quality of life. I'm suggesting a switch to normal 3° maybe 5 km before touchdown. According to my maths, 3° all the way gives a height of 1.05 km over me, whereas 5° until 5 km before gives a height of 1.57 km. 7° over those first 15 km followed by 3° would give a height of 2.09 km over me.

I saw this question about issues with steeper glide paths, energy management, etc.

Fuel consumption
My question is actually meant as stated: is it feasible?

Bianfable has chosen to raise the fuel consumption aspect as one reason why this would supposedly be a more environmentally detrimental approach to landing. Apart from the fact that this is not germane to my question, in fact this argument also doesn't appear to stand up: instead of descending from cruise height at a constant 3° it would be perfectly possible to descend at 2° initially, then 5° (if this is feasible, e.g. using speed brakes and landing gear), and then do the final 5 km at 3° for example.

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– Ralph J
Commented Mar 27 at 5:59

A descent procedure optimized to minimize noise is the Continuous Descent Arrival (CDA), where aircraft avoid leveling off during the arrival phase, which would come with increased thrust and therefore noise (and fuel consumption).

Continuous Descent Arrival (CDA) is an aircraft operating technique in which an arriving aircraft descends from an optimal position with minimum thrust and avoids level flight [...]

The objective of a CDA is to reduce aircraft noise, fuel burn and emissions by means of a continuous descent, so as to intercept the approach glidepath at an appropriate altitude for the distance to touchdown.

(SKYbrary - Continuous Descent)

The CDA profile until reaching the FAP is ideally not defined by any constraints on the procedure (STAR) nor does it use a fixed angle. It is simply based on aircraft performance1. The flight management computer (FMC) calculates the top of descent (T/D) point such that the aircraft will arrive at FAP at the correct altitude and with the correct airspeed using an idle thrust descent. 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 popular2.

Your suggestion of using a 5° descent until 5km is not feasible because that is the phase where the aircraft needs to reduce its speed quite significantly. You typically slow down to 250 kt at 10,000 ft (earlier) and then to final approach speed (~150 kt) during this phase. That is impossible to do while flying a 5° descent, even with the speedbrake fully extended. It is actually more realistic to fly a steeper final approach once the extended gear and flaps add more drag, which is what London City is doing (5.5° instead of 3° final approach). While 5.5° is too steep for many airliners, 4° to 4.5° is typically possible.

Another reason why reducing noise near Heathrow is difficult, is the incredibly complicated airspace around London. There are several major international airports in close proximity with conflicting arrival and departure routes and of course a lot of congestion resulting in delays and holding patterns, which makes it difficult to impossible to implement CDA, especially during busy times.

Average holding times were about 8.5 minutes at the beginning of 2014, with that figure now just over 7.5 minutes and falling as low as 6.5 minutes in August [2016].

(NATS - Heathrow holding times on the decline thanks to new technology)

1 Modern airliners have a glide ratio of ~20:1, resulting in a descent angle of only ~2.9°. Adding idle thrust on top will only reduce this angle further. Since airliners are optimized for minimal drag in clean configuration (no flaps/slats extended), this glide ratio has been increasing over time, resulting in shallower idle-thrust-descents.

2 Any increase in drag will always result in a deviation from the optimal idle-thrust-descent and therefore result in higher fuel consumption (e.g. because a level altitude needs to be maintained longer to start the descent later or because the previous descent-segment was flown at a shallower angle).

• "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? Commented Mar 25 at 10:08
• @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. Commented Mar 25 at 10:15
• @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. Commented Mar 25 at 11:09
• @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. Commented Mar 25 at 11:49
• @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. Commented Mar 25 at 12:42

The simple answer to this is clearly "yes, they could". It appears that the glide path could be made steeper through the use of flaps alone. Speed brakes (or even landing gear) are obviously used to remove kinetic energy through increased drag but flaps seem to do this too. In any event there are obviously many permutations for removing kinetic energy (e.g. at higher altitudes, as discussed, before starting on the 5° slope).

I didn't ask about the fuel consumption questions. However, contrary to what is argued by Bianfable and the apparent biases of most of the commenters who have responded to this question, the issue of fuel consumption does not appear open and shut.

An interesting paper at Research Gate, entitled "On the Influences of an Increased ILS Glide Slope on Noise Impact Fuel Consumption and Landing Approach Operation" concludes as follows:

Steeper landing approaches can lead to a significant noise and fuel reduction. Having regard to the sink rate limit of 1.000 ft/min below 1.000 ft height they can be performed only up to 4.0° glide slope angle. In addition a change of gear/flaps schedule is necessary to fly approach angles of more or equal 3.5°, which can reduce the noise benefit gained from the higher flight path.

Note the use of the word "significant".

Note also that this paper is concerned primarily with the whole final approach glide path (i.e. until touchdown). Part of the focus is on safety with steeper final (final-final) approaches: it concludes there isn't much of an issue with 3.5°.

My suggestion of "levelling-off" to 3° at 5 km from the runway also probably makes this concern moot. I'd be surprised if safety could be a deal-breaker with a suggested 5° until 5 km to touchdown, but ultimately don't have the expertise to say one way or the other, e.g. regarding energy management.

More research is clearly needed on the noise and fuel implications of such a two-slope or three-slope approach.

• There are some issues with 3.5 deg mentioned in the paper: "Operations in tailwind conditions on a 3.2° glide slope and a further increase to 3.5° may induce too high sink rates in piloted flight associated with the risk of a go-around." The way I understand it, the steeper descent allows less time to correct sink rates and increases the chance of go-around. Commented Mar 26 at 10:26
• @ROIMaison Tailwind, yes, not that surprising. Obviously certainly metereological conditions might make this sort of "composite slope" configuration less practical. In the case of LHR typically (not always) a tailwind (i.e. wind from the east) would result in a switch to "westerly operations" (planes landing from the west). Commented Mar 26 at 12:58