Ask The Pilot

[747sp]

My question is prompted by the Air India crash.
If the flaps are not used on takeoff how is the lift of the wing affected?
If there is an engine failure the other engine is strong enough to lift the plane. Can the engines provide enough power to overcome the lack of lift if the wings are not correctly configured?

 

[jb747]

My question is prompted by the Air India crash.
If the flaps are not used on takeoff how is the lift of the wing affected?

They're always going to be used to some extent. Generally the lowest setting gives you slats only. The next couple will give area increase at the trailing edge, and perhaps some rear curvature. After that the curvature increases dramatically, and they produce a lot more drag. These draggy settings are never used for takeoff, only landing, and in the event of a go-around, the flap is retracted away from them immediately.

With lower flap settings, you need to be going faster to produce the same lift. They're used on long runways mostly, where effectively you do your accelerating on the ground before getting airborne.

If there is an engine failure the other engine is strong enough to lift the plane.

The engines never lift the plane. All they do is push it forward. As long as you've got at least as much forward thrust as you have drag, you'll be able to maintain any speed. Add a little more thrust and you can accelerate.

Can the engines provide enough power to overcome the lack of lift if the wings are not correctly configured?

As long as the aircraft is above the stall speed, the engines should be able to accelerate it. If it you don't have enough power, then you may have to trade altitude for speed, which isn't something that's always possible.

To give you an idea of stall speeds (this is the 767-300 @ at its maximum takeoff weight):
Clean 188 kts
Flap 1 (slats) 155 kts
Flap 5 (slats and trailing edge area increase) 142 kts
Flap 20 (slats, trailing edge, and some curvature) 137 knots
Flap 30 (slats, trailing edge, and lots of curve) 127 knots

If the aircraft is incorrectly configured, the degree of error would matter. Flap 5 instead of 20 would be barely noticeable. Clean instead of 20, and it wouldn't get airborne, or if it did, it wouldn't be for long. Takeoff configuration warnings simply go mad with no flap, but they'd probably allow the wrong one, as long as it's a defined takeoff setting (i.e. not 30). I don't believe they took off without flap. One possible action has the non flying pilot retracting the flaps, instead of the gear after take off. The flap lever has a very solid gate, designed to make this sort of error difficult, but assuming you got past that, the aircraft itself would refuse to retract the leading edge devices (called auto gap on the 787). So, my feeling is that you'd still be in a situation the aircraft could fly away from.

 

[ChrisGibbs]

i was under the impression there was some logic that said that on take-off you couldn’t engage the auto thrust unless you had the correct flap setting?

 

[jb747]

i was under the impression there was some logic that said that on take-off you couldn’t engage the auto thrust unless you had the correct flap setting?

I don't recall that as a limitation (but I could well have forgotten). But, what is the correct setting? The Airbus was very definite about it being the same one that was entered into the takeoff performance page on the FMC. Apparently the 787 uses an equivalent check to allow completion of the electronic checklist, but the configuration warning only checks for the flap to be at one of the allowed takeoff settings. That's from a 787 pilot, so not first hand.

Beyond that of course, you can always take off without auto thrust.

 

[QF WP]

A post from Kevin Sullivan on LinkedIn regarding the SriLankan Air incident piqued my curiosity:



Link is here to report: Serious incident to the Airbus A320 registered 4R-ABM operated by Srilankan Airlines en route (Indonesia)

Was there a concern about flying without the pitot heater operational?

That certainly sounds like they were busy with conflicting ECAM’s - can our pilots comment?

 

[Rabs97]

Was there a concern about flying without the pitot heater operational?

Depends on the MEL. Boeing is quite restrictive, can’t really operate in icing conditions or visible moisture (pretty much given in that part of the world). Airbus may be different.

That certainly sounds like they were busy with conflicting ECAM’s - can our pilots comment?

Unreliable airspeed can cause haywire with warning systems, so it’s possible for overspeed and stall warnings to sound simultaneously. I’m not sure performing a stall recovery would be the correct action given no other stall indications, but the write up is pretty brief and I suspect more will be revealed in the final report. Certainly for me unreliable airspeed training emphasised correct pitch and thrust over every other warning.

The goal is to fly set pitch attitude and thrust settings only until you recover the speeds or until you land. It is an extreme handful, one pilot consumed with hand flying the exact settings and the other doing everything else. It’s not just for emergencies that you need two pilots, even a “normal” day can be quite a handful with two professionals up front.

Thankfully EASA ended their single pilot study, even in the cruise.

 

[jb747]

I’m surprised that any pitot heat MEL relief would be available at all. I wonder if it came with any caveats. I know that QF engineers attempted to use a similarly restricted MEL on a domestic flight, when the MEL said “no icing conditions”. They didn’t seem to understand that just because it’s 30º at the airport, doesn’t mean that’s the only temperature that the aircraft will encounter. The failure sequence listed above makes sense, given that they left with FO pitot off, and have subsequently lost the standby as well. Dual failure takes away most of the automatics, because you don’t want the FBW following a single spurious input (like MCAS did). Nothing in that failure sequence is outside of what is practiced. You should go through an entire career and never see it, but it is prepared for. That works until you get a strange outlier like AF447.

For very good reasons, Kev doesn’t have much faith in automation.

 

[MEL_Traveller]

Due to weather in SYD all day aircraft have been using the cross runway.

Noticed however that tonight’s QF11, an a380, took off on the regular, long runway.

If this is the case, why aren’t more aircraft using the longer runway instead of the massive delays at SYD? Or is the A80 better equipped for the SYD conditions?

Edited to add the VA65 to DPS also used the main runway. A 737.

Obviously up to the pilots, and it’s obviously safe. So why are most aircraft on the cross runways?

 

[jb747]

Due to weather in SYD all day aircraft have been using the cross runway.

Noticed however that tonight’s QF11, an a380, took off on the regular, long runway.

Because even with the headwind, 250 would be too short for the operation.

If this is the case, why aren’t more aircraft using the longer runway instead of the massive delays at SYD?

Because wind isn’t the only issue with the weather. Looking at the videos, the approaches are all in ‘actual’ IMC, which means that you’ll need greater separation between aircraft, and you’ll need to be careful of anything that interferes with the ILS (like a crossing aircraft).

Or is the A80 better equipped for the SYD conditions?

The 380 has a very good set of crosswind limits, appreciably greater than, say, the 737. Note, though, that the crosswind limit varies for takeoff and landing (consider the flight control you need to use if you lose an engine after V1) and also with the conditions. Contaminated runways (i.e. wet, snow, etc) all reduce the limits dramatically.

Edited to add the VA65 to DPS also used the main runway. A 737.

Winds aren’t constant. If you need a particular runway for its length, but the crosswind is an issue, waiting for the wind to drop below the limit is a viable strategy.

Obviously up to the pilots, and it’s obviously safe.

This is a leap, and is quite dangerous thinking.

It’s appreciably less safe than taking off into the wind. Many things involve some degree of risk. You train for it as much as possible, but that does not remove the risk entirely. I’ve seen takeoffs in the 767 and 747 in which almost full aileron has been required to keep the wings level. You don’t need much change at that point to run out of control deflection. In the engine failure case, if you’re already applying substantial rudder to remain straight because of crosswind, and then lose an engine that requires even more, you again run the risk of running out. People don’t generally run off the side of the runways in calm conditions.

Think of it this way. There is no risk of going off the edge at 0 crosswind, and 100% chance of doing so if you exceed the limit (by say 20%). You have the choice of 30% of the max crosswind, or 90%. Which one is safer?

So why are most aircraft on the cross runways?

Because their crosswind limits demand it, and it’s overall safer.

 

[MEL_Traveller]

This is a leap, and is quite dangerous thinking.

Thank you for the detailed reply!

Re the above my thinking is that a Qantas a380 captain isn’t going to make the decision to depart unless it was safe to do so.

I see the EK413 similarly used the main runway, but departing to the south, rather than the north as the QF11.

 

[jb747]

I see the EK413 similarly used the main runway, but departing to the south, rather than the north as the QF11.

That just means that the wind had swung a bit more to the north.

 

[Rabs97]

Another thing particular to Sydney is their Precision Runway Monitoring for the parallel runways. Means you can run a full arrivals program on both sets of runways in IMC. Basically you have two aircraft in IMC doing instrument approaches close together, with a dedicated Monitor frequency who’ll issue a breakout instruction if two aircraft get too close.

However YSSY hasn’t run this program for a while. Unofficially I was told they don’t like doing it in strong crosswinds as there’s a greater chance of overshooting final approach and causing a proximity alert.

Mostly they use the cross runway if wind exceeds 20kts, which can be a bit frustrating with the delays it incurs given most aircraft, even B737s have crosswind limits well above that (can vary up to 37kts dependent on things like surface condition, flap position, CoG).

Having said that the weather at Sydney yesterday truly was a shocker with crosswinds over 40kts at times.

 

[jb747]

Another thing particular to Sydney is their Precision Runway Monitoring for the parallel runways. Means you can run a full arrivals program on both sets of runways in IMC. Basically you have two aircraft in IMC doing instrument approaches close together, with a dedicated Monitor frequency who’ll issue a breakout instruction if two aircraft get too close.

PRM is not a feature.

However YSSY hasn’t run this program for a while. Unofficially I was told they don’t like doing it in strong crosswinds as there’s a greater chance of overshooting final approach and causing a proximity alert.

Pilots never liked it. You have to have another frequency dialed up, and someone has to listen to it. The procedure is not a normal go around, but something invented in an office. Go arounds are procedural, and that's what makes them safe. This wasn't.

It existed because the two 16s are not sufficiently far apart to allow independent approaches. As far as I can remember, it only existed in one other place.

Mostly they use the cross runway if wind exceeds 20kts, which can be a bit frustrating with the delays it incurs given most aircraft, even B737s have crosswind limits well above that (can vary up to 37kts dependent on things like surface condition, flap position, CoG).

It varies up to 40 knots (380). I don't recall it changing with flap or CofG. Surface condition, and visibility affect it dramatically. For most aircraft a max of 25 knots on a wet surface. 15 knots in low visibility. Landing and take off limits are likely to be different too.

20 knots is enough of a handful in some aircraft that QF limits FOs to that figure.