Delta 3508 Low Fuel Diversion - Questions

[simon001]

I have some questions about a diversion that took place on March 4 for Delta 3508, originally scheduled to fly from DTW to IAH, but diverted to OKC. My brother in law was on the flight.

https://flightaware.com/live/flight/DAL3508/history/20200304/1730Z/KDTW/KOKC/tracklog

The schedule flight time is listed as 3:23. Flightaware shows time in the air, on another flight that completed normally, as around 2:40. This flight was on a CRJ-900LR. The diverted flight was on a CRJ-900ER.

In comparing the diverted flight with a normal one, I saw that both aircraft completed their climbs to FL360 / FL340 in about 25 minutes. The normal flight had a level cruise speed averaging 400 knots, which it maintained for about 1:50. But the diverted flight had a level cruise speed of 320 knots.

The track log showed that the diversion took place after 25 minutes of level cruise.

By my calculations, the 80 knot difference in cruise speed would have taken the diverted flight an additional 30 minutes in for the 730nm cruise part of the flight, plus additional time during the climb and descent.

The normal flight took a track of 227 degrees to IAH for much of the cruise phase. I noticed that the diverted flight took a more Westerly path on a heading of 250 degrees from the get go. Flightaware does show a broad area of weather down South that day.

How much fuel reserve is such a flight likely to take? Might destination weather have been a factor in the decision to divert, which was taken only 25 minutes into the cruise portion of this 3 hour flight? Might other factors have been at play?

[neilki]

I have some questions about a diversion that took place on March 4 for Delta 3508, originally scheduled to fly from DTW to IAH, but diverted to OKC. My brother in law was on the flight.

https://flightaware.com/live/flight/DAL3508/history/20200304/1730Z/KDTW/KOKC/tracklog

The schedule flight time is listed as 3:23. Flightaware shows time in the air, on another flight that completed normally, as around 2:40. This flight was on a CRJ-900LR. The diverted flight was on a CRJ-900ER.

In comparing the diverted flight with a normal one, I saw that both aircraft completed their climbs to FL360 / FL340 in about 25 minutes. The normal flight had a level cruise speed averaging 400 knots, which it maintained for about 1:50. But the diverted flight had a level cruise speed of 320 knots.

The track log showed that the diversion took place after 25 minutes of level cruise.

By my calculations, the 80 knot difference in cruise speed would have taken the diverted flight an additional 30 minutes in for the 730nm cruise part of the flight, plus additional time during the climb and descent.

The normal flight took a track of 227 degrees to IAH for much of the cruise phase. I noticed that the diverted flight took a more Westerly path on a heading of 250 degrees from the get go. Flightaware does show a broad area of weather down South that day.

How much fuel reserve is such a flight likely to take? Might destination weather have been a factor in the decision to divert, which was taken only 25 minutes into the cruise portion of this 3 hour flight? Might other factors have been at play?

It was an operational diversion for fuel.
any number of other factors could have been at play. sounds like dispatch and the crew did a good job.

[MarkerInbound]

How much fuel reserve is such a flight likely to take? Might destination weather have been a factor in the decision to divert, which was taken only 25 minutes into the cruise portion of this 3 hour flight? Might other factors have been at play?

The US domestic rules require 45 minutes of reserve fuel. Plus fuel to an alternate unless the the 1-2-3 rule is met. From one hour prior to one hour after the ETA the cloud ceiling is at least 2000 feet and the visibility at least 3 miles. Looking at the weather that afternoon they probably had an alternate but it may have just been HOU so say another 15 minutes of fuel for a total of an extra hour's worth of fuel. Your other two questions are impossible to answer without talking to the crew and dispatcher so I’ll just say yes and yes.

[simon001]

Okay. Let me ask a "practice vs. theory" question on fuel:

If the forecast weather was ideal, both along the route of flight and at the destination, and the traffic at the destination expected to be light, might an airline pilot really only carry just 45 minutes of reserve fuel?

[jimjim1]

The normal flight had a level cruise speed averaging 400 knots, which it maintained for about 1:50. But the diverted flight had a level cruise speed of 320 knots.

The speeds displayed by the ADSB tracking sites are usually Ground Speed. They most likely had a headwind. Flightaware shows some weather in a band across the southern USA, presumably for the time of the flight.

[MarkerInbound]

Okay. Let me ask a "practice vs. theory" question on fuel:

If the forecast weather was ideal, both along the route of flight and at the destination, and the traffic at the destination expected to be light, might an airline pilot really only carry just 45 minutes of reserve fuel?

It's all about money. It costs money to carry extra fuel so it’s quite possible to release the aircraft with just 45 minutes reserve. In reality there would probably be a bit extra. Airlines have an immense date base of flight date. Say the noon DTW-IAH flight averages 18 minutes of taxi time. The release might call for 30 minutes taxi fuel. If the plane gets off in 15 minutes they’ve got a minute or two extra holding fuel. If they started with just 18 minutes taxi fuel and it took 20 minutes to get to the runway they’d have to go back for more fuel and that would also have costs.

[misd-agin]

Okay. Let me ask a "practice vs. theory" question on fuel:

If the forecast weather was ideal, both along the route of flight and at the destination, and the traffic at the destination expected to be light, might an airline pilot really only carry just 45 minutes of reserve fuel?

Legally you can. In reality that's not what happens. The company's I worked at had a minimum of one hour of fuel consumption at normal cruise altitude. Fuel flow at normal cruise altitude is close to the fuel burn you'd have while holding. But if you're going to be slowed down requiring flaps to be extended fuel flow will increase (eg, one aircraft mentions fuel flow increasing by 7% with flaps 1).

Diversion fuel is also predicted on a perfect plan via the filed route. No delays, no off track diversions, etc. That often doesn't happen in reality so guys add a bit of additional gas to their expected fuel burn to divert. My last diversion we were kept in the hold for another turn and then vectored for about 20 miles in the exact opposite direction that we wanted to fly. That burned up approx. 1,000 lbs of fuel that is not included in the planned release fuel. This is why Captains frequently divert slightly prior to the absolute minimum allowed.

What happens if the divert field is now backing up with inbound traffic? We were the 7th flight to divert into the airport on my last divert. It turns out that there was no other traffic but unforeseen delays and vectors could have added to more unexpected fuel burn. Which is why it's not uncommon, if the situation means that you'll probably diverting regardless how long you hold, to hear 'if you're going go early.' The first to land is often the first refueled and the first relaunched. We were the 6th of 7 to divert and the last to leave.