Operational question

[737ngtechQ]

Hi,
I'm a 737ng F/O and am a bit unsure of how temperature changes in the cruise affect indicated airspeed with respect to overspeeding and losing IAS.
I have referenced all my books and searched online but still can't quite get it straight in my head. Any thoughts would be appreciated.
I understand that the FMC is aiming for a Mach number (ie 0.78), and that the speed of sound is entirely dependant on temperature. So, for a constant mach number, at a constant temperature, a certain TAS results.
ie if you're maintaining M0.78 and the temp is -52 your TAS is 452
if you're maintaining M0.78 and the temp is -62 your TAS is 441
What I want to know is, what happens momentarily when you fly from a parcel of warm air into a parcel of cool air, on your PFD. Inertia is the thing that causes a momentarty over or underspeed obviously.
By my reckoning, flying into a cold parcel of air should cause the momentary overspeed but I have observed some captains being wary of warm air in this respect. Have I got it backwards?
Thanks in advance for your thoughts.

[Takeoff OK]

Maybe I'm missing something here (and it wouldn't surprise me if I was) but in 2500 hours of jet transport time, it has never been an issue. I've never seen this fluctuation you're speaking of.

The question itself is interesting enough, though.

[737ngtechQ]

Hey,
I'd be interested to know what type you fly to see if it's a 737ng thing because I had never experienced it either with about 2000hrs on other type jets. I've done about 500 on the ng and am starting to realise that if you're up high, and fly into a front or jetstream, it can get a bit exciting quite quickly
Thanks for your reply, all thoughts are appreciated.

[gonnabeapilot]

Just curious if you fly all the NGs or simply the -800? The -800 can definitely take you for a ride up at altitude however I've spoken to friends at Westjet and apparently that is something that is unique to the -800 and they do not experience the same type of thing with the -600/-700s.

In regards to your question, perhaps you're just looking at things from a different perspective than your Captains. They may be worried about an over-speed but not in the sense that you're thinking of. Flying in to warmer air will close up the amber bands as performance decreases. If you are cruising at a higher mach number up around your max altitude, there is a chance that the top amber band will come down to meet your cruise speed creating a situation where you're over-speeding... just not in the traditional sense where you've passed .82 and are into the bricks. It's the same sort of reason to never use the speed-brakes in cruise to prevent an over-speed... only use thrust management. As soon as the speed-brakes deploy, you load up the wing and the window can close right up on you making a bad situation much worse. Hope this helps.

Cheers,
gonnabe

[True North]

It's the same sort of reason to never use the speed-brakes in cruise to prevent an over-speed... only use thrust management.

Interesting. That is contrary to the guidance Boeing gives. Boeing specifically advocates use of speedbrakes to control overspeed at altitude.

[gonnabeapilot]

The Boeing Flight Crew Training Manual states the following about the use of speed-brakes in cruise:

Overspeed... ...If autothrottle corrections are not satisfactory, deploy partial speedbrakes slowly until a noticeable reduction in airspeed is achieved. When the airspeed is below VMO/MMO, retract the speedbrakes at the same rate as they were deployed.

It's interesting to note that this is the only time that Boeng ever recommends partial use of speedbrakes as the FCTM also states:

Use of speedbrakes between the down detent and flight detent can result in rapid roll rates and normally should be avoided.

The way it has been explained to me is that the intention of these two statements is for speedbrakes to only be used as a last resort in cruise and even then only AFTER an over-speed condition has already occurred. If you are sitting in cruise all fat, dumb and happy, and you see the speed trend arrow take off towards an over-speed hauling out the speedbrakes will only make things worse, especially if you deploy them quickly. The only action of the pilot should be to assist the auto-throttle in reducing thrust and not to deploy the speedbrakes until long after you've entered the bricks. In my experience flying -800s this action has always been more than enough to prevent an overspeed condition in cruise. Now I'll be the first to admit that my knowledge of high speed aerodynamics is weak so I'm definitely open to hearing a different interpretation.

[North Shore]

?Entered the bricks? What does this mean, please?

[The Fish]

"The Bricks"
The red and black blocks that represent the overspeed regime, on analog gauges it's commonly referred to as the "barber pole".

Cheers

[gonnabeapilot]

Over-speed and stall are displayed as alternating red and black blocks on the speed tape. As a result it looks like a bunch of red bricks stacked on each other, hence "the bricks".
edit: beaten to it

[whipline]

I wouldn't concern yourself to much with temperature increases and decreases. Watch the effects of wind speed and direction on indicated airspeed/Mach. Don't be afraid to manipulate the throttles yourself. Dont climb above optimum and keep lots of margin between high and low speed. Dont try to out climb weather or turbulence. Descend, speed back and go around it. The QRH for overspeed calls for attitude adjustment, not speed brake deployment. Speed brakes increase your angle of attack, which is the last thing you want at high altitude. Unless of course you want the high/low "bricks" to merge which would become a bad day quickly.

[737ngtechQ]

Gonnabeapilot, I just fly the -800 so it may be as you describe.
Whipline, I hear what you're saying about wind changes, I'm pretty comfortable with that and will often assist the A/T by taking an extra few % off the N1, or sometimes when it's just maintaining 4-5kts above scheduled speed I'l use speed intervene to wake the A/T up and regain the target speed before deselcting it.
My interest in the temperature issue is probably 70% accademic and 30% operational. (I like to have a topic on the go most of the time to increase knowledge).With that said however, there is quite a strong feeling that temp changes play a significant role in overspeeds on the -800 and I would like to understand it properly.
The QRH says

Reduce thrust and, if needed, adjust attitude to
reduce airspeed to less than Vmo/Mmo.
- - - - - - - - - - - - - - - - - - - - - -

Personally I would never raise the speedbrakes in the cruise.
Reducing thrust is the best option I think, especially as adjusting attitude will result in an altitude bust for the situations I'm thinking about.
But my question is not so much how to deal with an overspeed in the cruise, but how rapidly changing temperatures can induce it. I'm still uncertain as to which is worse, flying into cold air or warm air, someone came up with a good point about the amber bars converging with the warm example.
Any more thoughts?
PS thanks for your input.

[Brad MacKay]

737ngtechQ, you are right on.

The aircraft auto throttle system may be slow to
correct the conditions you are anticipating.

Temp is very important in cruise, it let's you know if you
have passed thru the trop in a climb or decent.

Many times our cruise altitudes end up busting into and out
of the trop's warm/cold air boundaries with the aircraft performance
changes just as you explained.

If you are wondering why you are getting a wave activity
similar to a mountain wave when ther are no mountains
for a 1000 nm, you are probably flying very close to or in
and out of the trop.

I find when planned at high heavy altitudes (ie: 1.3 G manuever) invicinity
of the trop that I am cautious of the AT over reacting to
commanded speed reductions when entering the warm air
as the AT when left to it's own at times waits to long to add
thrust after the speed reduction has been achieved leaving
you with that sinking feeling of not enough thrust and to much
drag.

[737ngtechQ]

Ahhhh into and out of the trop....nice, hadn't thought of that, thanks. I pictured temp changes more like fronts or sometimes jets.
Can anyone answer me this..... If I was flying along maintaining M0.79 at optimum FL, and the SAT went from -56 to -46 over just a few seconds, what would I see on my speed tape? What would happen to the amber bars and also the IAS?

[Brad MacKay]

Decrease.

Try this link.

http://www.smartcockpit.com/flightops

Cheers

[737ngtechQ]

Thanks a lot for that link Brad, thats gunna have me busy for years!
When you say decrease, I'm managing to confuse myself again because my tech books tell me ;
"Indicated airspeed, however, at a given altitude and mach number, is always and conveniently the same. For example, an aircraft in LRC at FL 350 at M0.83 will have an IAS of 283kts irrespective of outside air temperature."

So the second I fly into that warmer air, the magenta bug depicting the Mach number is going to start moving up the screen i reckon, which gives underspeeding initially......sound fair? Then, with the application of thrust the a/c claws its way back to the magenta bug, which although still indicating the same mach number, is actually a higher TAS but the same IAS.
And the reverse for flying into cold air.
Now the only thing I want to get my head around is what the amber bars do with these rapid changes in temperature. Any ideas?

[TopperHarley]

As you fly into a warmer air mass, technically the IAS/CAS should not change. The airspeed indicator measures the "dynamic pressure", which mathematically is 1/2*rho*V2 (rho= air density, V= true airspeed). It's the TAS that will change, and in this case it will increase. The air density (rho) will be lower, associated with the warmer temperature, but the TAS will be higher. There is no overall change in the dynamic pressure.

Another way to think of the Airspeed Indicator is like this. If you had a CAS of 200 kts at Sea Level and a CAS of 200 kts at 20,000ft, the dynamic pressure in both cases is the exact same. The airspeed indicator doesn't "know" which factors (air density, or, True Airspeed) are making the diaphragm expand to show the same CAS. From our Private Pilot days, we know that for a given IAS, TAS increases with altitude due to the lower air density. The same thinking can be applied with a warmer, less dense, air mass.

Mach Number is TAS/Local Speed of Sound. The LSS varies directly with temperature. In the above example, as you fly into warmer air at a constant IAS, the TAS will increase. But the higher temperature is also associated with a higher LSS. So the overall effect on your Mach Number will depend on the relative change of the TAS and OAT.

I dont think temperature alone will affect the lower amber/red speed tape. The lower range of the speed tape is an indication of your stall speed. Stall speed (in terms of Indicated), doesn't change with altitude/temperature. With high performance jets, the IAS technically does increase slightly with increasing altitude. But in your example the aircraft is at the same altitude, and only the OAT is changing. So Im thinking that the Indicated Stall Speed won't change, and the amber/red speed tape will stay where it is.

In terms of the upper part of the speed tape, which reflect Vmo/Mmo, Im guessing that since the OAT is higher, the Local Speed of Sound will also be higher. Therefore the mach effects of drag, compressibility, etc, will occur later compared to a colder air mass which has a lower Local Speed of Sound. Drag will increase significantly as you approach the Speed of Sound, so Im guessing your Mmo will be higher in the warmer air mass compared to the colder one (since the speed of sound is higher).

This is all kind of tricky since we're not really used to thinking of this kind of stuff in terms of OAT alone. Generally we're more concerned with the effects of altitude on Stall Speed/Mmo (and as a jet pilot you know all about Coffin Corner).

Im not sure if my analysis is all correct, so take it with a grain of salt. I think a question like this is best asked in the Tech forum on www.pprune.org.

[slob driver]

Hi there,

You may or may not wish to do this, but what about trying different TOC OAT on your perf init page before going flying. What I mean is put in for example -42, see what the optimum altitude is, and then change it to -50 and look at what your optimum is. A backdoor method of seeing the effect of OAT on your buffet boundaries I would think. Does this make sense, or am I completely off beat here with regards to what you're looking for?

However, I am quite certain that the amber bars will close up in warmer air. I believe warmer air equals lower optimum altitude. I remember in annual a couple of years ago, we discussed the situation you are referring to. One of our a/c went through some warm air, and very quickly the amber or red bars closed up. Can't remember if it was the lower 1.3 g amber band joined up from the bottom, or vice versa, or if both upper and lower bricks joined up completely, but needless to say it caught the attention of the crew!! Especially the speed at which it occured.

In Bulfer's fmc user's guide, he states that the most critical NG with regards to buffet margin is an 800 or 900 at high altitude in warm air;thrust limitations.
I would interpret that as warmer air =higher chance for low speed buffet situation.
Frankly, in the NG I am not as concerned about an overspeed situation as much as an underspeed situation. I have heard anecdotally that dive testing has been routinely done to .86 on the NG with little or no effect with regard to Mach buffet. As of now, I've got between 4000-5000 hours on the NG and have never been CLOSE to that. Maybe .825, but no more. Low speed though? A lot of mountain wave following the divide will make you sweat the low speed. When in doubt, just go down if you can, imo. The buffet opening versus fuel burn is a no brainer in the NG. At least within reason.

Would you agree or disagree?

[737ngtechQ]

Hey Slob,
yeah the underspeed is more serious really because you have to leave your flight level in order to regain the speed if you get too far behind the curve. Even at Max Continuous it can take a minute or more to regain five knots in some circumstances. All in all the trick is to not fly above optimum if it's bumpy and keep an eagle eye on things but I'm still interested in understanding it. I'l try your idea out tomorrow and get back to you.

[True North]

Hi there,

You may or may not wish to do this, but what about trying different TOC OAT on your perf init page before going flying. What I mean is put in for example -42, see what the optimum altitude is, and then change it to -50 and look at what your optimum is. A backdoor method of seeing the effect of OAT on your buffet boundaries I would think. Does this make sense, or am I completely off beat here with regards to what you're looking for?

It's not just buffet boundaries. Depending on the temperature you enter you could be bumping up against the thrust limit as well. If memory serves, you will hit the thrust limit at altitude somewhere around ISA +15. Unlike the FMC in other Boeings, the installation in the NG doesn't tell you if you are thrust limited or buffet (weight) limited.

[True North]

All in all the trick is to not fly above optimum if it's bumpy and keep an eagle eye on things but I'm still interested in understanding it

So I take it you are allowed to fly above OPT ALT in your operation. I find that interesting, we never did.

Just curious why you would ever want to?

[737ngtechQ]

There is no SOP forbidding flight above optimum in our company.
It is crew discretion as to what FL they use.

Just curious why you would ever want to?

Most of the time we will be at optimum or below, but , if it's smooth and the choice is between 3000' below optimum or 500' above it, with little prospect of getting a different level for the next three hours, most will take the 500' above it.
We can potentially get a bit shy on fuel at the other end if we get stuck below optimum as well. Its not normally a difficult decision, if it's turbulent then we look for another solution.

So I take it you are allowed to fly above OPT ALT in your operation. I find that interesting,

Yes we are. Things like this are interesting because to me it seems overly prescriptive to make a rule against it, so it shows I am influenced by what I'm used to and that there is more than one way to skin a cat. I imagine situations where flying 500-1000 ft above optimum would be the best option available to a crew and it seems a shame that the company would take that option away. Your thoughts?

[whipline]

The difference between optimum and max on the -800 is around 1700 feet based on a C of G of 26.2. I've personally in 6000 hrs on it never witnessed the airspeed get low but I have had the Mach rumble occur on two occasions. Wait until you see what the auto throttles do. I'd much rather descend then climb. For fun ask your dispatch to run a flight plan 2,000 feet lower then originally planned. You'll be amazed at how little difference there is. Not really worth it IMHO.

Impressive after only 500 hrs on the -800 your wondering why it can't hold a speed in cruise and have the aptitude to select CON in the FMC!!

[True North]

There may be circumstances where flying above optimum might be the best option, but IMO they are few and far between. As whipline pointed out, the fuel burn difference between OPT and OPT-2000' is quite minimal, around 2%. Interestingly, the difference in fuel burn between OPT and OPT+2000' is also around 2%. Fuel burn aside, the further you go above OPT the narrower your margins get and even when things are going just fine, it can get ugly in the blink of an eye. Don't ask me how I know.

All that said, I do understand when you're flying for the airlines these days fuel is very dear and you seldom have too much. I lived in a different world. All my time in the NG was on a BBJ and as anyone who has flown corporate knows, its a very different game. The people I flew around were very accustomed to getting what they wanted and if we were going to LAX, that is where they expected to go. Spending an extra $500 or $1000 dollars to carry a few thousand pounds of extra fuel was not an issue. In fact it was normal for us, it was insurance. Better to hold for an hour and get into LAX than have to divert to Palm Springs after 30 minutes.

In the end it's what you are comfortable with. If your company SOP allows it then just make sure you know what you are getting into if you do it, which it seems, is what you are doing with your questions. Good on ya.