kidicarus5897

Is there a rule of thumb for what TAS to fly when experiencing a given headwind or tailwind. I usually see, and it makes sense that, we are usually dispatched to fly MSC with a headwind and .76 or so with a tailwind to maximize efficency, but I can't figure out what formula they use. Anyone have an idea? Curious because with winds different than forcast for the flight plan, it might be adventageous to increase/decrease TAS accordingly to save fuel.

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RBower

I doubt they use any formula, just seat of the pants type planning. Headwind? Full blast. Tailwind, .76 sounds good! More detailed, efficient planning is out the scope of this company.

If you want to maximize mileage use the specific range readout in the performance page. But I know of no rules of thumb and we don't have any wind trade off data. Remember though the idea is to fly the airplane at minimum cost, not minimum fuel. Time costs money too.

kidicarus5897

I should have stated that if time is not a factor, it might be benifical to slow with a tailwind

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yasir1212

Not too long ago, I had a tailwind of about 95 knots or so (forget where I was going) and I got speed my discretion from ATC, pulled it back like to Mach .65 and saved about 400 lbs of fuel over the course. We were showing arriving about 25 mins. early, and we still showed up like 10 early to the gate. Gotta love how quite the airplane is at those speeds.

erj-145mech

Not to mention that if you get to the hub too soon, you'll end up waiting for a gate anyway.

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nixon

From "Handling the big jets" aka bible for Cathay interviews take your filed TAS - half the tail wind or headwind and add or subtract to the TAS. For example flying westbound with a TAS of 420 with a 60 kts headwind - 420+30 (1/2 of 60) - fly at 450 (not as likely as subtracting for tailwind. The essence: spend as little time as possible in headwinds and as long as possible in a tailwind. Clear as mud?

Yariv

As was said already, we don't have all the necessary data to give you a good answer. but we can do better than cathay.

your goal is to arrive at the destination with maximum fuel remaining.

we've traditionally used LRC speed as the most efficient cruise speed, but this is only true with no wind. btw LRC speed itself does not change with wind.

LRC is not useful for your purpose in wind because its goal is to maximize range. our range is fixed, we want to minimize fuel consumption instead.

Here are some generic charts to help the discussion:



as was said before, fuel is not the only cost, time is also a cost. not only in that we have to maintain a schedule (let's say that's not relevant to this question) but also in that the longer the trip takes, the more fuel used... so time is fuel, and there is a balance point where going any slower will COST fuel rather than SAVE fuel:



wind does not affect TAS or mach. if you want to take wind into consideration, shift the mach number in the two charts at the top (7.8 and 7.9 - headwind=lower resulting mach, tailwind=higher) and the result will be that in the bottom chart(7.10), optimal mach number changes very little! what changes is that the whole banana shaped curve moves down to a lower cost for a tailwind, and up for a headwind. These are typical numbers for a 757 (couldn't find an RJ one, but the point is that the change is very small)



so how do you make this useful? for academic purposes, no change is required, or you can change by .1 mach for 100kts of wind... for real world purposes, schedule does matter, so my technique is (probably the same as everyone else's...) to increase in a headwind with fuel remaining as a guideline for how fast to go, and to decrease in a tailwind, with ETA as a guide but no less than LRC TAS. if you go slower than LRC, you're wasting gas.

the cathay trick is probably a general rule of thumb for aircraft without FMS. I can't think of a reason to use it, if you have a fuel remaining and ETA estimates that allows you to adjust your speed to exactly what you need. if you have enough gas, the adjustment will be the FULL wind value, not half. and if you were filed close to LRC, you wouldn't decrease by half the wind to a value below LRC.


AERO - Fuel Conservation Strategies: Cruise Flight

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