Ask The Pilot

[AviatorInsight]

@AviatorInsight any trips to Hobart today or planned for tomorrow? The winds around the Airport have been and will be really strong and gusty. Some flights cancelled this morning and inbound returned to originating port Not saying it’s going to be a problem, should be a lot of fun.

Unfortunately no. I’m on days off before my first HND trip on Friday. Yes I finally opted in given that they are now ending, figured I might as well do a longhaul flight again for old time’s sake.

 

[D747]

Why would aircraft manufacturers allow a pilot to rotate the aircraft past the angle where the tail hits the ground, while there is still weight on the wheels? Airbus limits angle of bank and pitch in normal flight, why not do the same when the aircraft is still on the ground? Would an aircraft's speed be affected by the friction of a tail scrape, so preventing this would also help in the above scenarios?

 

[JohnM]

As a gross error check I use the take off weight - 20 to get the V2 (take off safety speed). For example, if the take off weight is 70T then the V2 should be around 150kts (give or take a couple of knots).

That was one aspect of the three examples above that I wondered about. Were they so far off the mark that it should have reasonably triggered a "This doesn't look right." type of response?

Or is it so finely balanced that it's not so easy to be attuned to that?

 

[Quickstatus]

"This doesn't look right."

If i may inject a slightly different perspective. In my non aviation work, there is checking and there is checking. Where errors seems to occur is when the checking process is just a "going through the motions".

Real life experience:
2 people need to verify the bag of blood before it is administered - they are supposed to read out aloud the check items when giving blood - patients arm band correlating with label on bag of blood amongst other items. 2 people checking can read the label correctly but not recognise the difference. The patient armband says "Mrs Jones". The bag of blood says "Mrs James" . Both checkers (both very respectable and experienced nurses) correctly read out the arm band "Mrs Jones" then correctly read out aloud the label on the bag of blood as "Mrs James". Tick.....Blood given. Luckily the transfused blood was compatible with the patient.

Inconceivable? Never.....

 

[RSD]

Sometimes you look at how far along in a process that a major mistake happens before it is picked up and you just realise that sometimes things just escape. A classic example was when Woodside were building their Pluto LNG project up in Dampier about 12 years ago. The flare stack there is extremely tall, one day a vac student engineer from Perth looked at the flare stack with his head tilted to one side and said "is that thing cyclone-rated?" - a quick check of the drawings resulted in an Oh cough! moment and consultant engineers being engaged to strengthen them urgently.

 

[jb747]

After so many take offs you tend to have a good idea of the speeds that should be set for the different weights.

That was very much the case with the 767 and 747, where the data was calculated from charts. BTW, you got pretty good at them, and could extract the complete data in about a minute.

But, the advent of coughpit computers doing the numbers gave the performance people a chance to incorporate many more factors into the mix. The aim was to get the best performance outcome possible, but one of the effects was that it removed a lot of our 'feel' for the data. If you can imagine the range of performance outcomes using derate of anything up to 40%, coupled with flap settings that varied dramatically, and the numbers start to make less 'sense' than previously. Taking LA as an example, you could have three departures over the course of a roster, all off the same runway (24L), at similar weights. Even similar conditions. And yet, one might go with flap 1, and other 2, and the third might use 3. The excess runway off this runway was often a big, fat, zero.

This is from the 380....an actual flight in mid 2015. The number we looked at as our overall check was the 'stop margin'.

 

[AviatorInsight]

This is from the 380....an actual flight in mid 2015. The number we looked at as our overall check was the 'stop margin'.

Seeing as I can't use take off data from KLAX, I've also done a take off performance calculation using the same data. (HW5, 24ºc, Q1010).

I thought your speeds looked familiar...

 

[jb747]

The AB flap 3 setting is probably closer to Boeing flap 20. It’s just an arbitrary 1, 2, 3, and full. And whilst the Vspeeds are similar, the weight differs a bit.

Is 79 tonnes the max?

 

[AviatorInsight]

The AB flap 3 setting is probably closer to Boeing flap 20. It’s just an arbitrary 1, 2, 3, and full. And whilst the Vspeeds are similar, the weight differs a bit.

Is 79 tonnes the max?

Yep it is.

I ran those numbers at Flap 5 because that is our most common setting. If I left it at optimum then it would have been flap 1 with a V2 of 166.

I actually needed flap 25 to get out of Hamilton Island the other day. That's always fun as the aircraft levitates off the ground rather than rotate.

 

[jb747]

Is flap 5 just leading edge, or is there some trailing as well? Presumably you'd be getting some area increase at the trailing edge with one of the early settings.

The 380 also had an automatic setting. Flap 1 was actually just slats, but "1+F" was flaps and area increase at the rear. If we selected F1 on the ground, we'd actually get "1+F". After takeoff, it would automatically retract to "1" about 15 knots before retraction speed. And when on approach, it would also stage through 1 to 1+F by itself. All to do with the speed limits for the various settings.

 

[AviatorInsight]

There is trailing edge there at flap 5 but the slats will only extend to the first stage (sealed). Depending if it's an SFP (YI* series, etc) or not (VBY, VOU, etc) then the slats will only go to full (slotted) at F30 and F10 respectively. This is why if we need to slow down on approach, flap 10 works great on the older models.

 

[Saab34]

AV, will you try and get a Haneda flight in before they disappear?

 

[serfty]

AV, will you try and get a Haneda flight in before they disappear?

See post at the top of this page:

Ask The Pilot

@AviatorInsight any trips to Hobart today or planned for tomorrow? The winds around the Airport have been and will be really strong and gusty. Some flights cancelled this morning and inbound returned to originating port Not saying it’s going to be a problem, should be a lot of fun...

www.australianfrequentflyer.com.au

 

[JohnM]

Just posted. Some memorabilia for @jb747:

 

[Quickstatus]

What is the MTOW minimum control speed in the air and the design take off speed for the 380 and 737? thanks
( I suppose these are specified at some standard air temperature, pressure)

 

[D747]

Why would aircraft manufacturers allow a pilot to rotate the aircraft past the angle where the tail hits the ground, while there is still weight on the wheels? Airbus limits angle of bank and pitch in normal flight, why not do the same when the aircraft is still on the ground? Would an aircraft's speed be affected by the friction of a tail scrape, so preventing this would also help in the above tail strike scenarios?

@jb747 @AviatorInsight ?

 

[jb747]

Why would aircraft manufacturers allow a pilot to rotate the aircraft past the angle where the tail hits the ground, while there is still weight on the wheels?

Sorry, I missed this one.

Most tail scrapes actually occur after the aircraft is airborne, so a weight on the wheels switch wouldn't help. They happen because the rotation rates are too high. The target attitude for V2+10 will almost certainly be enough to give a scrape...so you don't want to get there too soon. About 2.5-3º per second will be as fast as you want. Conversely too slow a rate will lead to excessive runway use, and degrade the initial climb performance.

But, there are cases where you need minimum unstick (perhaps so that you don't run into the bloke who has entered the runway without a clearance) and in that case the tail scrape is the least of your problems.

Airbus limits angle of bank and pitch in normal flight, why not do the same when the aircraft is still on the ground?

They don't really. They simply make it harder to achieve. The limits are there, but basically 'soft'. Near the ground, AB are not in their normal inflight law, and there are no limits.

Would an aircraft's speed be affected by the friction of a tail scrape, so preventing this would also help in the above scenarios?

It would have to have some effect, but I'd suspect that the high AoA would give more drag.

 

[jb747]

What is the MTOW minimum control speed in the air and the design take off speed for the 380 and 737? thanks
( I suppose these are specified at some standard air temperature, pressure)

How long is a bit of string? It depends upon the amount of derate that you're using (amongst other things), which goes some way to explaining why you don't push the power up after an engine failure on a derated take-off.

The nominated numbers though...
Vmca1 120 knots
Vmca2 144 knots.

 

[Quickstatus]

why you'd don't push the power up after an engine failure on a derated take-off.

Increased power will affect directional control and rudder will not have enough authority to counteract at that speed? Effectively increasing Vmca?

But why would you increase power when the performance calculations already take engine failure on takeoff into account?. Which is V2.

 

[jb747]

Increased power will affect directional control and rudder will not have enough authority to counteract at that speed? Effectively increasing Vmca?

But why would you increase power when the performance calculations already take engine failure on takeoff into account?. Which is V2.

V2 is simply the speed target. You aren’t necessarily climbing all that well. More power is good, as long as it doesn’t complicate any lateral control issues.