Why you *should* step outside the cockpit occasionally

[Bruce (a.k.a. brian747) 142]

I know, I know. Me too.

 

I have always gone along with the accepted wisdom that outside views are for wimps and tourists: yer actual real simmer is totally focused on the flying experience, and only the cockpit counts.

 

But today I decided to make a quick video (no rendering or clever stuff needed, just a straight recording as I went along) of an assumed temperature take-off at Heathrow (the Aerosoft version, in case you're wondering). I made it because I have been getting increasingly concerned recently about why I was using up so much runway — until I viewed the video, which instantly provided the answer. Slowly but surely I have been getting slower on the rotation. (No longer, though: identifying the problem is most of the way towards a solution).   

 

The three-minute 720p video can be found here if you could bear to watch it (and, if you would be so kind, sympathise):

 

https://www.youtube.com/watch?v=fl4RZXRBICA&feature=youtu.be

 

The point is, I suppose, that like a golf swing that slowly develops problems that you don't notice until they're pointed out to you, flying technique also needs an additional pair of eyes from time to time.

 

So, it turns out that out of the cockpit views can have their place, too.    

 

Cheers,

 

bruce

a.k.a. brian747

 

[hifly 925]

A little tail dragging there, maybe some trainer wheels at the back until you get the hang of it. Also noticed no wing flex.

 

Call me a wimp but I often take a peek outside to admire the eye candy of the Q400, PYB and others.

[brett 2,315]

I tend to stick to the cockpit but like Geoff, how could you not want to ogle the aircraft. Not to mention during a long flight to check out some scenery from an extended look.

 

I also like to use the recorder for landings on a new purchase or difficult approach to make sure I am doing thing correctly.

 

It's been a long time since I have flown a big bird but something didn't look right during your takeoff. Once you rotated it seems as if the plane was stuck to the runway for too long and that lack of that lift one gets when reaching speed during rotation. I am probably talking out of my rear end but that's what it looked like to me. 

[Quickmarch 488]

@Brett,

 

No, I believe the time from rotation to lift off (~10 seconds) feels a bit long. I don't fly the 747, so I can't offer a learned opinion - just a gut feel. More data is needed: V speeds, Load, etc.

 

It's a cool video Brian.

[Bruce (a.k.a. brian747) 142]

Hi guys!

 

And thanks.  

 

@Geoff

 

The lack of wingflex is the result of using the default 747 for the video ( <grin> I wasn't thinking that it might be posted at the time I made it). IMHO, a tail strike is typically the result of a rotation that's too fast, not too slow — and since I'm using PSX (the professional simulation of the 744) any tailstrike would have been announced loudly and clearly in the cockpit!  

 

And yeah, I agree, I really should start to enjoy the view more.  

 

(Those who dislike seeing an aircraft in pain, please look away now). Here's a tail strike on the real thing, incidentally:

 

 

<wince>  That's gotta hurt!

 

(Picture taken during Boeing's original high attitude take-off tests, as reproduced in D.P. Davies's classic book "Handling the Big Jets").

 

@Brett

 

"Sticking to the runway" is just what it looks like, I entirely agree. It was an assumed temperature take-off on a very warm day at Heathrow, so maybe that also has something to do with it....      

 

@Quickmarch

 

The rotation speed looks slow to me, too: the FCTM calls for rotation at 2 - 3° per second, which I thought I was doing, but she also doesn't seem to be getting the lift I would have expected following V_R (flaps 20) — which is why I'm also wondering about the assumed temperature setting. Mind you, once she's in the air and accelerating along the CPT5J SID then then she's fine. 

 

But the main learning point for me (I think) was that the PSX cockpit in particular is so detailed and immersive that it's easy to stay "head down" — I need to look around more and smell the aviation fuel. Following the Flight Director is all very well, but sometimes it takes an outside view to indicate that there's a problem.

 

OK, it seems to be time to refresh my take-off technique.... Errors creep in — perhaps it's an age thing (again).   <sigh>

 

Many thanks for your kind comments, guys!   

 

Cheers,

 

B.

 

 

 

[mutley 4,498]

Hello Bruce, I was agonising at the rotate, worried you would run out of asphalt!    

 

Nicely done, a few breaths of fresh air often have beneficial effects. 

[hifly 925]

IIRC back in the days when I might have pulled back too hard on rotation sound effects would have told me that my backside is on the asphalt. I would pause and replay the take off just to enjoy the fireworks.

 

@Bruce, that's what I like about this hobby, it's a constant learning curve. Now get back into the sim and I want to see ten more practice rotations before lunch.

[jaydor 345]

I love being a wimpy tourist or the screenshot section would need a transplant. 

 

Enjoyed watching the video, knowing I am not alone.. 

 

You could rename it a "Tail of Woe".. 

[allardjd 1,853]

It was an assumed temperature take-off on a very warm day at Heathrow, so maybe that also has something to do with it.... 

 

 

I suspect that had a LOT to do with it.

 

At the risk of putting words in QM's mouth that he didn't intend, I think he meant that the rotation was initiated at too low an airspeed for the AC weight and environmental conditions, i.e. he was talking about airspeed, not the rate of rotation in degrees per second.

 

If you manage to un-stick the nose wheel too soon, before there's sufficient airspeed to get you the lift you need at the correct angle of attack (something less than what will cause a  tail-strike) the extra induced drag* is going to dampen your acceleration and lengthen your takeoff roll.  

 

All that business of calculating Vspeeds is has a lot to do with that.  V1 and V2 are all about acceleration, available runway length and stopping distance, but Vr is all about at what airspeed the AC will be ready to fly at the particular AC weight, and the atmospheric conditions (temperature, barometric pressure and relative humidity) at that particular place and time.  I think that you are rotating too early for that set of circumstances and QM is right - look again at Vspeeds, particularly Vr.

 

John

 

*:  You are making lift when you raise the nose, just not enough, so it is induced drag, not parasite drag. 

 

 

[Bruce (a.k.a. brian747) 142]

Hi John, and thanks for your contribution which has, I believe, enabled me to identify the problem!   

> "... the rotation was initiated at too low an airspeed for the AC weight and environmental conditions..."

If I may say so, I don't think that was the cause, purely because of the extreme accuracy of the PSX sim (good enough for a friend of mine who lives a few miles away and just happens to be a recently-retired 744 Training Captain to enthuse about) and the fact that I rotated as soon as the PNF called it. The PSX FMC is considerably more refined than PMDG's, say, and in the eight months since the PSX release I have learned to trust its predictions with considerable confidence.

 

I have now been vlying 744s almost exclusively for well over fifteen years (starting with PS1 (for which I wrote "The Big Tutorial"), then following a short flirtation with PMDG's 744 with PSX since its release eight  or nine months ago), so with any luck — and I say this tentatively, you understand — most of the more elementary errors are unlikely. I hope.   

BUT....  

The factor which you have now prompted me to remember (and that I had previously overlooked  <sigh>  ) was that for the last couple of weeks or so I have been using a recently-released and still under development PSX utility which gives better assumed temperatures than TOPCAT — at least, it does *most* of the time (for the engines I use, and on dry runways). As you say, it just happened to be an unseasonably warm day (25°C at Heathrow in April doesn't happen too often) and I suspect that such unusual conditions may be somewhat outside the boundaries of the utility's capabilities as yet. An assumed temperature that was too high would perfectly account for the unusually slow acceleration and sluggish lift-off, and also the fact that in spite of applying the usual stick pressure the rotation was perceptibly slower than normal. Had I applied any more elevator, however, I would have been at the 11-12½° attitude where a tail strike does occur, so that wasn't an option. (As you probably know, at the recommended flaps 20 lift-off attitude of 10° the minimum tail clearance is a slightly scary 40 inches (102cm) above the runway, which is something that I'm always aware of). Happily, the TORA at Heathrow's 09R is 12,743 ft (3884m), which is surely the only reason why I didn't overshoot the end of the runway.     

I do worry about the strange results I sometimes get from TOPCAT, but I think I'm going to have to go back to using it from now on — until the new utility is... more refined, shall we say.   

Many thanks indeed for prompting my erratic memory to recall the real reason, John!   

All the best,

B.
 

[allardjd 1,853]

Ummm  - OK.  Not sure I understand yet.  Was the WX wrong, or was Vr wrong or was the power setting wrong (EPR?) or was everything right, just not what you expected?  I'm sure there are other possibilities too but just don't understand what was the problem.

 

John

[Captain Coffee 2,030]

TYVM John. I was wondering if I was the only one to not spot the problem despite:  "...view(ing) the video, which instantly (did not) provided the answer.

 

So, what ARE you doing wrong Bruce? Why so slow on the rotate? Stowaways?!

 

P.S. I TRIED to do a flight without leaving the cockpit last night. Couldn't do it, the views from up on the wing are just tooo nice to stay inside.

[Quickmarch 488]

It's interesting to see the outside view and I'm going to try a quasi-split screen takeoff in the FSX stock 747 (744). I can do this with a VC view in the main window (I do have to see the speed tape) with a locked spot viewport opened in the top left portion of the screen. Unfortunately, taking a video of this takeoff is beyond my meager abilities.

 

Parameters might be:

 

Long R/W such as 26L at CYVR 

TOW = 875K (for some reason FSX lists the MTOW at 880K on the kneeboard but will only allow 875K in the Fuel and Payload panel - start up and taxi fuel? Seems a bit extreme.

Temp = 70F (21C) @ 29.92"

Wind = Calm

Elev = CYVR is at sea level

ASDA = 11,500 (TODA=12,500) Note: FSX-SP2 CYVR lists Runway Length = 11,962 (assumed to be TORA) This from ADE.

RW = Dry

Flaps = 20 (FSX shows 17%)

Trim = 7.5 nose up (this is the FSX default position so I'll leave it I'm unfamiliar with this aircraft)

V1 = 155

VR = 171

V2 = 181

 

Note: I haven't paid any attention to the nav gear other than to set speed and altitude restrictions in the A/P.

 

Let's see what happens......

 

 

Rotate!

 

This aircraft takes forever to respond. I rotated at 171 and didn't see any movement from the nose until way past V2. 

 

 

 

Here she is just coming unstuck (or as fast as I could hit the Pause button). The over-run looks a bit damp.  I think I was a bit too slow on the rotation. Gonna take some practice. I didn't get the nose up fast enough and kept her on too long.

 

Check out the PFD - 236Kt @ 40' and redlined N1. Positive rate! It took everything I had to get the gear up and the throttles back so as not to bust maximum speed. Flies nicely, though. Might be time to spend some of that hard earned AH dosh and really up my profile at QuikAire.

 

I should also take some notes on getting video.

 

 

[hifly 925]

March, in FSX in the Options dropdown there is a Flight video option, never use it but it's there. BTW I hardly ever fly at MTOW but if the runway is short I spool up to 50%  holding it on the brakes, release the brakes and ease the throttles up. 

[Bruce (a.k.a. brian747) 142]

Oh dear, it appears that whilst trying to make things clearer I have made things worse - apologies, guys.

The problem was an incorrect (too high) assumed temperature, exacerbated by an unusually hot day at Heathrow. This had the effect of reducing the take-off thrust commanded by the TOGA button to a figure which was insufficient to permit normal acceleration and climb out. (So the EPR was wrong, John).

The generic performance offered by the default FSX 747 is not remotely to be compared with the highly-tuned performance figures built in to PSX (unless <*cough*> some idiot screws them up by providing a wacky assumed temperature) I fear.

Incidentally, you *do* realise that the aircraft in the video is merely an FSX puppet being commanded by PSX, right?

Cheers, and thanks,

B.

[Captain Coffee 2,030]

Just tried this in CLS's 747-200 Martin Air cargo variation.

 

Flaps on the 3rd position (20%)

Held brakes till N1 hit 50%, release brakes

Took foreeeeever to get up to V1 and was nearly out of runway it seemed,

but V2 hit fast after, rotated sharply up and was off at 180Kts (175?) indicated.

 

Max fuel, Nearly MTOW, Stock Fair Weather at Travis AFB, Fairfield Ca.

 

 

Identical setup, but with the stock 747-400 Boeing Livery: 198KTs IAS at wheel separation from ground.

 

Not all 747's are created equal?

 

 

EDIT: ah, and now I see that the above means squat to the circumstances because I have no idea how to/where to "set a TOGA" command...let alone how to Assume a temperature...

I can set an autopilot to let me take a leak in flight, but other than that I'm all VFR hand flying these gizmo laden birds.

 

Cheers.

 

[allardjd 1,853]

Understand EPR.

 

Didn't understand that Boeings had TOGA button - I don't fly Tube-o-Chairs. Something to do with auto-throttles, I guess? I thought that was just a Busboys feature.

 

Didn't understand about the Play Station setup but kind'a do now.

 

What is "assumed" temperature? Is that the air temp that's used for calculating V1, V2, Vr, EPR, etc? I can see that could be the best guess for what the temperature is going to be a half-hour from setting up the FMC or something like that.

 

Good lesson here for me - I'm learning a thing or two - always good.

 

John

[mutley 4,498]

For clarification, Bruce is flying the Aerowinx Precision Simulator Ten (PSX) and FSX is providing the graphics, more in Bruce's review here.. http://www.mutleyshangar.com/reviews/bc/psx/psx.htm

[bilirubin 50]

Being new to the flight SIM world (less than a week) and trying to learn to fly using a keyboard whilst I wait for my joystick to arrive in the post the video looked great to me.

 

Do Microsoft make a SIM for a JCB to help dig a Cessna 172 out of the bank at the end of a runway because it would be a great help

 

Mike 

[jaydor 345]

Do Microsoft make a SIM for a JCB to help dig a Cessna 172 out of the bank at the end of a runway because it would be a great help

 

Mike 

 

Arr, you can now award yourself the same T-Shirt as I have.. 

[Bruce (a.k.a. brian747) 142]

@John

OK, you asked for it....

Airliner engines being a humungously expensive item (especially when you've got four of 'em on board ) there are various measures which are deployed to increase their life by, basically, trying not to push them too hard, since wear increases exponentially with temperature.

So when it comes to taking off in a large airliner, full power is only ever used on the relatively rare occasions that the circumstances require it (for example, a high airport on a hot day). But for the majority of the take-offs the accountants that sit on every airliner Captain's back dictate that full power is *not* used; instead only sufficient power to ensure a safe take-off should be applied (which clearly means that the take-off run will be longer, and the subsequent initial climb-out slower, so you'd better get your sums dead right! Unlike some.... <*cough*> ).

(Naturally, it's entirely the Captain's decision whether he uses full power or not — but he'd better be able to justify that decision if he decides to do so, that's all...)!

There are two ways to specify a reduction in engine power for take-off:—

1. Derated take-off

This is the old-fashioned and simplest method, and it basically derates (reduces) the engine's power by a fixed percentage such as 10% or 20%. It's OK, and is still used from time to time, but it's not as flexible as method 2.

2. Assumed temperature take-off

Basically, you lie to the FMC about the outside temperature, telling it that outside it is significantly hotter than is in fact the case. The FMC responds by selecting a lower thrust setting for take-off, and so you get a similar effect to method 1 but with more scope for fine tuning. You can get the required temperature to achieve the optimum thrust setting that is good for this runway under these conditions by consulting tables, of course, but since there are an awful lot of variables, these days it's more often done either on the pilot's EFB laptop or else you transmit a request to base for the right figure, which comes back to you shortly afterwards.

(In practice it's also possible to use methods 1 and 2 simultaneously, but I'm trying to keep this explanation as brief as possible, so I'll just talk about assumed temperature take-offs from now on).

In this picture you will see that I have selected an assumed temperature of 62°, although the OAT is only 20° (the FMC has calculated the resulting EPR as 1.53). I've also selected CLB 2 derate for the climb, just to make the point that the two methods can be combined:




When you release the brakes at take-off and push the throttles forward to the upright position (calling for around 70% N₁) you then watch the engine parameters on the EICAS display to ensure that they're all stable at that figure, and when you're happy about that you push a TO/GA button (they are conveniently placed behind the thrust levers for engines 3 and 4 so that you can easily find them without looking, since your hand is still on the throttles at this point). Pushing a TO/GA button engages autothrottle and causes the FMC to enter thrust ref mode and spool the engines up to the FMC's pre-calculated take-off power figure — THR REF is annunciated on the PFD (and you check for this, of course). In the real aircraft with motorised throttles the levers will then move forward to the calculated setting for you, leaving you to concentrate on the various stages of take-off. Incidentally, if you haven't pressed the TO/GA button by the time the aircraft reaches 50 kts the throttles won't move and autothrottle can't be engaged until you're above 400 ft AGL — just another little gotcha in the real aircraft (and PSX too, of course).

You leave your hand lightly on the throttles until V₁ is called, when you remove them (because this aircraft is now taking off no matter what!). At V_R you rotate at 2 - 3° a second to the target body attitude of around 15 - 18°, ensuring that the aircraft lifts off prior to reaching 8° of pitch on the Flight Director — since a tail strike *will* occur at 11° if you're still on the runway. Oh and during the take-off run be sure to keep the aircraft straight and on or near the centreline too, while you're at it.

So what with one thing and another take-off is a pretty busy time, which is why it's nice to have the take-off power precisely set for you, since that's one less thing to have to worry about.

The above is a simplified sketch, and there's obviously a lot more I could drone on about with all this, but you're one of the good guys, so I'll shut up. If you're at all interested in looking into the topic a little more deeply, I can point you to two useful sources of information:

a) There's the Boeing (pdf) document, which you can find here: http://www.theairlinepilots.com/forumarchive/quickref/reducedandderatedthrust.pdf

or....

b) you can view a video about the 747-400 TAKEOFF REF page on the CDU (made by a certain recently-retired BA Training Captain who lives a few miles down the road from me) here:

 

http://youtu.be/Zd0eK2Oq9rs

I hope that helps a little,

Cheers,

bruce
a.k.a. brian747

[allardjd 1,853]

Excellent - got it. So assumed temperature sounds like it is a conservatively high OAT value, chosen for calculating EPR for takeoff.

 

What tells you you've gone too far, i.e. selected a temperature too high, giving you an EPR that won't give you enough power to take-off in the runway available? Does it feed back a "runway required" value during the calculation phase that you can compare to the runway available?

 

John

[Bruce (a.k.a. brian747) 142]

Hi John!

 

> "So assumed temperature sounds like it is a conservatively high OAT value, chosen for calculating EPR for takeoff."

 

Yup, that's it. BUT as I said, there are lots of other variables too — runway length, runway slope, flap setting, aircraft take-off weight, you name it. Which is why the preferred route is via software rather than using the tables. Having said that, if you check out Peter's video he gives an example of the BA tables in use at KSFO rwy 28L (there's a separate table for each runway and each flap setting, so, as you can imagine, using electronic methods also saves carrying around tables the size of a young telephone directory, as well).

 

> "What tells you you've gone too far?"

 

Well that's why you're making your list and you're checking it twice (at least): AFAIK the only factor that can give you a clue that there's something off is if you see that the suggested EPR figure doesn't accord with what your experience suggests would be right for that runway. Although in some of TOPCAT's wackier moments I have also been alerted by the fact that it has suggested an assumed temperature that just doesn't seem reasonable for the conditions.

 

No pressure....   

 

Cheers,

 

B.

 

[Quickmarch 488]

@John

OK, you asked for it....

clip..............

2. Assumed temperature take-off

Basically, you lie to the FMC about the outside temperature, telling it that outside it is significantly hotter than is in fact the case. The FMC responds by selecting a lower thrust setting for take-off,

 

Sorry for the (dumb?) question, Brian. I'm not a practiced airliner simmer, so sometimes I've got to have my hand held. I'm sure all is well, but to me, the above statement is backwards.

 

Normally, more thrust, or some amount (in piston aircraft usually close to max) for a longer period, is required on a hot day. Lying to the FMC, by giving it a higher than real temperature figure, should result in higher thrust being delivered to meet the requirements of the hot day. Where am I missing the point?

[Bruce (a.k.a. brian747) 142]

Hi John (that's t'other John)!   

That's an excellent (and far from dumb) question — the concept of using an assumed temperature (a.k.a. flex thrust) definitely seems odd to everyone at first, believe me, and it's certainly not an easy one to "get your head around". Furthermore, I'm no expert at engine parameters, so TBH I'm not sure if I can explain it very well — but I'll give it my best shot.  

In general, all jet engines are guaranteed to provide the specified thrust at full throttle position at a given temperature (ISA + 15, or 30C) — but if the temperature rises beyond that point the air becomes less dense, and so less thrust will produced. Since we're looking for a way to cause the FMC to provide us with less thrust, telling the tin brain that the outside air temperature is much higher than is in fact the case will cause less power to be commanded by the engine.

(Notice, incidentally, that this applies for take-off only, and that the subsequent climb gradient will inevitably be reduced).

Which is the short version, but I can already imagine you still wondering why lying to the FMC doesn't cause it to provide *more* power, in an attempt to somehow compensate. As you said "...should result in higher thrust being delivered to meet the requirements of the hot day. " But in this case, as the saying goes, "that's not the way it works", in that no compensation is involved (at least, not in an obvious way), it's just the engine behaviour which is being manipulated. So here's a longer explanation, which might perhaps (?) make it slightly clearer (hence my apologies if parts of it are redundant):   

Our starting point is that a jet engine works by taking in air molecules at the front, accelerating them, and pushing them out of the back. Furthermore, denser air contains more molecules for a given volume; and Newton's laws tell us that the more molecules (and thus the greater mass) of air you can shove out of the back, the more thrust you will get. (I reckon we're all singing from the same hymn sheet so far).

Since hot air is less dense than cold, it also follows that if we rotate the N₁ fan at a constant speed (100%, say), the amount of thrust created will decrease as the temperature increases (thereby decreasing the density of the air going in). Also a no-brainer.   

But the crucial part has to do with that stuff about the N₁ fan, in that —

Entering an assumed temperature that is higher than the real temperature makes the engine produce the rated thrust for the given temperature.

In other words, we're not trying to *compensate* for the higher temperature, we just want change the engine's (and engines') behaviour, to reflect the supposed higher value. As I mentioned above, most engines are 'flat rated', so that if the engine is rated for 50,000lbs of thrust this is what you will get — but only up to ISA +15. The result of entering a temperature above ISA +15 is that the engines will then command a lower N1, which corresponds to the lower thrust that you would get at that higher temperature.

What you said about needing more thrust at a higher temperature is spot on, it's just that we're not in the business of wanting it to compensate for the higher temperature, we simply want it to perform as it would in that scenario.

 

<sigh>   Although, reading through what I've written, I'm still not entirely sure whether I've managed to explain it with any clarity, but hopefully it might have been some slight help?   

Cheers,

B.