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jb747

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Haha not that quite advanced. But there’s an extended motoring to prevent bowing of the N2 rotor. This is done automatically and fuel won’t be introduced until EGT is below 135° even with the fuel control switch to idle.
Still fairly warm, though I guess it's difficult to get cool with short turnarounds. Our target was 100º.

The risk if the initial temperature is too high, is that you'll get what's called "instant light-off", and will get rapidly increasing temperature and pressure in the core. That can lead to a hung start.
 

AviatorInsight

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Why is MCAS required at such high Mach?
Having a high angle of attack can come at any speed. It's all got to do with relative airflow. An aircraft can be descending and still have a high angle of attack. So MCAS has just got to do with getting the nose down and below that high threshold. The high speed means it's available throughout the entire flight envelope.
 

jb747

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Having a high angle of attack can come at any speed. It's all got to do with relative airflow. An aircraft can be descending and still have a high angle of attack. So MCAS has just got to do with getting the nose down and below that high threshold. The high speed means it's available throughout the entire flight envelope.
I think we need to separate some of our terms here. And I'll start by saying that I'm still concerned about MCAS, and don't see why it is needed outside of a fairly limited range. It is a form of angle of attack protection, but because Boeing have basically 'cheaped out' it doesn't allow the degree of fine control achieved by a proper FBW, in which you can fly the aircraft right on the AoA limit very accurately. I'd be interested in seeing its behaviour in a GPWS/terrain event.

Angle of attack (alpha) - the angle between the wing and the relative airflow.
Rigging angle - the angle between the wing and the fuselage. It's what caused aircraft to fly with the cabin very slightly nose up in the cruise. In large part its chosen for approach and on ground behaviour reasons.
Indicated airspeed (IAS) - the airflow you'd feel if you stuck your hand out the window. Variations called 'EAS' and 'CAS'.
True airspeed (TAS) - IAS corrected for the difference in the atmosphere. In simply terms, if the air is half the density, you'll need to go twice as fast to show the same IAS. The upshot is that at sea level IAS and TAS are basically the same, but as the altitude increases, the TAS for a given IAS also increases.
mach number - aircaft TAS expressed as a percentage of the local speed of sound. This is limiting at the upper end because various nasty effects start coming into play above the .85 to .9 speed range. In practical terms, mach number provided an upper limit on speed at high altitudes, but is largely irrelevant at lower levels.

So, how do you have a "high angle of attack at any speed". Well at low level you've got two options. If you're trying to maintain level flight, and you slow down, your AoA will increase. Keep slowing, and you'll eventually reach the stall AoA. Alternatively, you can apply some roll, and start to turn the aircraft. To maintain level you'll need to increase the g loading (as part of it will now be dedicated to making you turn, you'll need more overall to counter the 1 g of normal level flight). You increase the g load by pulling more and more backstick, which increases the AoA. Stall speed (which is associated with a more or less fixed AoA) increases. At 2g (a 60º angle of bank turn), it will increase by 41%. At 75º, you'll be generating 4g, and the stall speed will have doubled. So, if your 737 has a clean stall speed of 180 knots, it would have gone up to 360 knots at 4g.

In the cruise (at, say FL370) , you're limited by mach number. If you have a cruise speed of .83, that will give you an IAS of about 270 knots. Your TAS will be about 480 knots. If you apply the wind to your true heading and TAS, you'll end up with the track and ground speed. That's what gets you from place to place, but doesn't actually make the aircraft fly.

Now, at our .83 cruise at 270 kias, we're about 90 knots above the clean stall speed. But another number that comes into play is the minimum drag speed, and as it will be around 250 knots, we are only slightly faster than it. The aircraft won't immediately do anything nasty if we slow below min drag, but the issue is that at high altitude you won't have sufficient power to accelerate above it again, so the aircraft will start to slow down, and even with maximum power applied will continue to do so. If you're foolish enough to keep it up, you'll eventually end up with stalling becoming a consideration. The only solution is to trade height for speed.

So, lets put turning back into the equation. If, in the cruise, you roll into a 60º of bank turn, we'll need to generate that 2 G to maintain the altitude. Min drag speed, and stall speed will both increase by that 41%, so we're now well below min drag (now around 350 kias) and our stall speed has now jumped to about 255 kias. As we're so far below min drag, the aircraft will quite rapidly decelerate back onto the stall. Yuk. Because of the effects of turning on these numbers, in airliners are generally limited to 30º, and the autopilot will restrict its own turns at high level to below 20º.

The upshot of all of that is that the stalling AoA is never all that far away, and it doesn't really take much to get there, so some way of protecting the flight envelope becomes desirable. In the past this was up to the pilot, and then we had various forms of assistance with stick shakers and/or pushers, and eventually the ultimate form of protection in the form of FBW. But, there can be a reason for flying back near the stalling AoA, for that's where the most lift is produced. For example, perhaps an aircraft on approach runs into a microburst. Not only will it need maximum power to get out of it, but it will need to be flown right back at maximum AoA until it is in the clear. In something like the 747, you'd pull back until the stick shaker activated, then relax the pressure just enough to stop it. Holding exactly on the point of activation isn't easy. In a FBW aircraft you pull full backstick (all the way to the stop) and hold it there. The FBW system will continuously and aggressively activate the elevator to hold it on the cusp of stalling (they'll have picked an exact angle to use in the development). But the FBW systems never use the trim system for this. I don't know how MCAS would respond in this situation (AV?) but it is a case where any system that reduces your ability to get more AoA would seem to be a negative.

Stalling. There are two types of stalls that are mentioned in aviation.

An jet engine compressor stalls when the airflow through it is disrupted. This can be caused by damage from a birdstrike. Pulses of flame and banging are associated with this. The engine is not on fire. Generally the engine will be shut down. They can also be caused by airflow disturbances from extreme manoeuvering. Not normally an airliner problem, but the engine will normally recover by itself if the power is reduced to idle, and then advanced once normal flight has been resumed. Airliner engines in reverse can be prone to this, and it generally self recovers once selected out of reverse. They can also stall during start, and this will lead to an aborted start. You don't stall engines for fun, as it can be very damaging.

An aerodynamic stall happens with the airflow over the wing breaks away, and the flow becomes turbulent. At that point the wing mostly stops making lift, but does make lots of drag. It happens at about 15º angle of attack. Remember that the angle of attack is not the pitch attitude of the aircraft. Whilst stalling airliners isn't a great idea, it's a basic skill that's taught to new pilots, and many of the aerobatic manoeuvers you see at airshows involve going well past the stall.

Some aircraft have a particularly nasty charactistic in which the turbulent airflow from the stalled wing goes back over the tail plane, and diminishes its ability to give the nose down input needed to recover from the stall. Aircraft with T-tails are prominent here.
 
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Having a high angle of attack can come at any speed.
I thought MCAS was supposed to counteract the extra pitch up moment in the Max compared to the Non Max during a high acceleration condition from an increase in engine power - especially starting from a low TAS

But at high Mach, can you still get the same pitch up moment?. I thought it would be difficult to have the same acceleration with an increase in engine power at high Mach?
 

AviatorInsight

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I thought MCAS was supposed to counteract the extra pitch up moment in the Max compared to the Non Max during a high acceleration condition from an increase in engine power - especially starting from a low TAS

But at high Mach, can you still get the same pitch up moment?. I thought it would be difficult to have the same acceleration with an increase in engine power at high Mach?
No. Not to do with the pitch up moment. The normal speed trim system can handle this just fine, otherwise every take off at low gross weights we would have MCAS activating to help?

MCAS was designed because during the test flights of the stall there was a point where the control column pressure reduced. Meaning you could now hold on to the controls easier when it should get harder to hold on to.

So MCAS was designed to pitch the nose back down and it’s this that got the aircraft certified again.

As far as the high Mach question goes, if you’re in a dive and you let the aircraft accelerate you’re could be at a high Mach if you let it go long enough. Then if you had idle thrust, any advancement of the thrust levers will cause a nose up tendency and is part of the recovery.

At low level, the air is more dense so you will get a faster engine acceleration compared to high altitude.
 

AviatorInsight

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I think we need to separate some of our terms here. And I'll start by saying that I'm still concerned about MCAS, and don't see why it is needed outside of a fairly limited range. It is a form of angle of attack protection, but because Boeing have basically 'cheaped out' it doesn't allow the degree of fine control achieved by a proper FBW, in which you can fly the aircraft right on the AoA limit very accurately. I'd be interested in seeing its behaviour in a GPWS/terrain event.
Yes and I agree with you. AoA indicators are a “user option” and of course our company have not gone with with this option (either in the NG and I’m guessing the MAX as it wasn’t in the sim). So how can we determine our AoA? With other instruments. We have a flight path vector, pitch limit indicators, and aircraft pitch bars. The difference will be the distance between to draw out max alpha and fly the limit if we need to.

How the MCAS handles this in a GPWS event, I’ll let you know once my cyclic comes around in it.
In a FBW aircraft you pull full backstick (all the way to the stop) and hold it there. The FBW system will continuously and aggressively activate the elevator to hold it on the cusp of stalling (they'll have picked an exact angle to use in the development). But the FBW systems never use the trim system for this. I don't know how MCAS would respond in this situation (AV?) but it is a case where any system that reduces your ability to get more AoA would seem to be a negative.
Yes, MCAS reduces your AoA to below the maximum threshold (predetermined value that is not known to us). However it doesn’t take away the ability to have maximum elevator authority if you need to take it back to the high AoA after it has pitched the nose back down. Even then once it has done that, it will re trim back to the targeted trimmed airspeed.
 

jb747

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I thought MCAS was supposed to counteract the extra pitch up moment in the Max compared to the Non Max during a high acceleration condition from an increase in engine power - especially starting from a low TAS
The system has been subject to a fair bit of “mission creep”. If you‘ve followed the Mark Forkner story at all (the Boeing Tech pilot), one of his emails expressed surprise at the extension of MCAS from the low speed to high speed regime. Basically I think Boeing realised they had an issue, and then thought they’d just modify an already designed system for an increased role. That had the effect of making it less useful or safe for it’s original role, and we know how that ended.

They seem to have gone a bit MCAS mad too. They added it to the USAF 767 tankers, and whilst the 767 may have had a strong pitch couple, it was nothing that normal airline ops couldn’t handle, and so strikes me as doubly useless for an air force role. A bit of ‘cost plus’ perhaps.
Yes and I agree with you. AoA indicators are a “user option” and of course our company have not gone with with this option (either in the NG and I’m guessing the MAX as it wasn’t in the sim). So how can we determine our AoA? With other instruments. We have a flight path vector, pitch limit indicators, and aircraft pitch bars. The difference will be the distance between to draw out max alpha and fly the limit if we need to.
Oh, you can get near to max alpha, but my point is that in extremis, the ability to pull full back stick, and have the system give you the exact limit, may be life saving. I believe there was a major airline that had a very near miss in windshear at HK, in an A340, that only barely recovered, simply because it was flown right to the limit. Manually, no matter what the instrumentation, you’ll never quite match that, and to be honest, the average airline pilot won’t be that close at all. Boeing have played with partial FBW systems in some aircraft, but if they’re going to keep airframes for decades, I don’t see why they haven’t gone all the way. You don’t need a new airframe….an FBW 767 would be nice!

As for AoA gauges…they make a nice backup for airspeed displays, and will give very accurate speed control on finals. But few pilots know how to use them, and that would be another ‘cost’ as they’d have to train for it in the sims. Again, we all know how airlines feel about that.
 

Fergo747

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Pilots, interesting scenario on Friday evening leaving LHR. We got to the runway threshold (27L) after taxiing from T3 gate 13 when the captain announced that we would need to sit there for 5 or so minutes to burn off some fuel to meet MTOW. We then sat there and heard the engines spool up and down for the 5 or so minutes before we got underway (the captain also mentioned this would be the case as if we sat there at idle we would be there for ages).

I guess my question is how does this (being over MTOW) happen? I understand that conditions change (e.g. wind direction, runway) that can impact on assumed fuel burn between gate and take off, however from what I could see there was no major change with aircraft taking off / landing the same direction all evening.
 

Stevo.1702

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Pilots, interesting scenario on Friday evening leaving LHR. We got to the runway threshold (27L) after taxiing from T3 gate 13 when the captain announced that we would need to sit there for 5 or so minutes to burn off some fuel to meet MTOW. We then sat there and heard the engines spool up and down for the 5 or so minutes before we got underway (the captain also mentioned this would be the case as if we sat there at idle we would be there for ages).

I guess my question is how does this (being over MTOW) happen? I understand that conditions change (e.g. wind direction, runway) that can impact on assumed fuel burn between gate and take off, however from what I could see there was no major change with aircraft taking off / landing the same direction all evening.
Ramp weight limit is higher than the Takeoff weight limit. Ramp weight is basic weight, plus all fuel, cargo, and pax. Essentially what you would be when the final door is closed.

Fuel burnt maneuvering to the runway ideally will bring the aircraft from ramp weight down to MTOW. I imagine LHR can either have very long or very short taxi routes based on traffic, parking, and runway allocation. In this case, it seems like their taxi fuel allowance was not fully burnt and they reached the runway still above MTOW.

There would definitely have been some negotiation with ATC. High-powered runs to get the fuel flow up likely wouldn't have been approved on a taxiway. Although sitting on a runway for a few minutes would have required some coordination.
 

AviatorInsight

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Who would get their butt kicked because of this?

I thought this was quite funny actually. The fact that the FO had gone through the motions of a regular line flight, and it probably wasn’t until they were settled in the cruise and then started the usual conversation of “so how long have you been here”, or “how long you been on the (insert aircraft type) for” before they realised.

The FO might have also been wondering why the captain hadn’t asked him any questions.

Even once checked to line, airlines will usually employ a no green/green policy. Meaning a new captain to the fleet won’t be able to fly with a new FO on the fleet until either one has met the minimum experience requirements.
 

AviatorInsight

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Pilots, interesting scenario on Friday evening leaving LHR. We got to the runway threshold (27L) after taxiing from T3 gate 13 when the captain announced that we would need to sit there for 5 or so minutes to burn off some fuel to meet MTOW. We then sat there and heard the engines spool up and down for the 5 or so minutes before we got underway (the captain also mentioned this would be the case as if we sat there at idle we would be there for ages).

I guess my question is how does this (being over MTOW) happen? I understand that conditions change (e.g. wind direction, runway) that can impact on assumed fuel burn between gate and take off, however from what I could see there was no major change with aircraft taking off / landing the same direction all evening.
That‘s all it is, the fact that the taxi fuel was not burnt prior to commencing the take off run, which leads to aircraft being overweight for take off. This can happen for a number of reasons and we used to coordinate with LAX ATC quite a bit on the 777.

Some nights we would get a dream run to the runway where we hadn’t burnt off all of the taxi fuel so ATC put us on an adjacent taxiway until we were ready. Other nights we just pushed back at the wrong time and got stuck behind everyone and ended up burning all of the taxi fuel + most of the contingency before we even left.
 
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I recently watched a Y-tube where the FO was PF and it seemed she engaged the Auto Pilot about 30 seconds after rotation. It was certainly less than a minute.

Obviously every take off has different factors affecting the flight but is there a "best" time to engage the AP and do airlines have a minimum criteria to be reached first ?
 

jb747

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I recently watched a Y-tube where the FO was PF and it seemed she engaged the Auto Pilot about 30 seconds after rotation. It was certainly less than a minute.
There is no ”minute” limitation. Boeing specifies 250' for the 767 and 747, and Airbus 100' and 5 seconds as the minimum. So basically complete the rotation, and then you can engage it.
Obviously every take off has different factors affecting the flight but is there a "best" time to engage the AP and do airlines have a minimum criteria to be reached first ?
Whilst some airlines have gone somewhat mad with their autopilot policies, QF left it up the the Captain. The only place where autopilot engagement was a mandatory part of the departure procedure was London, as it made tracking that little bit more accurate and was less likely to set off the noise bells on the ground.

Personally, I’d engage it anywhere between that 100' and 10,000'. As a generalisation, I probably engaged it around the time the aircraft was clean and the after take off checklist done. But if the SID was complex, or the weather was poor, or it was the middle of the night and we’re all tired, it would go in earlier. Oh, and on checks. Anyone who shows a check pilot their manual flying skills is an idiot.

Which reminds me of a tale… I’d been to London via Singapore. The FO was scheduled to have a check between Singapore and Melbourne on the way home. So, at the start of the trip, he asked if he could have the London - Singapore as practice. That set off the alarms as most people didn’t actually care, and certainly didn’t need “practice”. So, in the briefing for departure from London he states that he’ll engage the a/p at 5 or 6 thousand feet. That’s against the SOPs so I tell him he‘ll be engaging in accordance with them (i.e. 250'). The response was that “he doesn’t like flying via the a/p, and is just as accurate manually”. Yeh, I’ll bet. At the Singapore end, he disengages the a/p very early in the descent (above FL200). Because he now has to fly and manage, and he’s at 100% brain capacity just flying, he does very little managing. He was, at least, giving the SOs a great example of how not to do things. In the debrief I told him that his manual flying is pretty poor at best, and that if he’s going to pass his route check then he’d better engage the a/p as soon as possible, and keep it engaged as long as possible.

On the night of the departure, the weather has thunderstorms all around Singapore, and multiple heading changes are needed to keep clear of them. Mister manual flying got way behind the aircraft, and I took it off him. Instant fail.
 
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