Narrative:

We were ferrying the aircraft after landing gear malfunction on the previous leg. In preflight planning; I checked the dispatch release. The operation modes for climb; cruise; and descent were 250/grdn. At pushback; the FMS indicated that we had 13;300 pounds of fuel on board with a landing fuel of 7;800 pounds. About 20 minutes into flight; I checked our fuel burn rate. We were burning 7;000 pounds of fuel per hour. Meanwhile; the FMS was still showing that we would land with 7;800 pounds of fuel. We updated the fuel on board in the FMS [but there was] no change. Something didn't add up. I did the mental math; we were burning 7;000 pounds of fuel per hour; we had 8;500 pounds of fuel on board and we had more than an hour to go. I did the math again; how could this be? Not only would we not land with 7;800 lbs of fuel; at this rate; we could/would run out of fuel before reaching our destination. While the first officer and I were working these numbers; dispatch sent a message asking us how we were doing on fuel. I gave him the fuel update and asked him what the highest altitude was we could fly. He told us we could go to 20;000 ft but that the fuel numbers were for 12;000 ft. We climbed to 12;000 ft. The fuel numbers still did not look good to me. We started looking for the nearest suitable airport to land. I called dispatch; we agreed on a diversion airport. When we were 16 miles from the airport; the FMS was showing fuel on board 4;770 pounds and the landing fuel 4;800 pounds; which is obviously not possible. Thankfully; we landed safely. Some example of the fuel numbers from FMS; fuel on board vs. Miles to go vs. Landing fuel: 13;000 pounds 375 miles 7;800 pounds; 6;850 pounds 99 miles 6;000 pounds; 6;400 pounds 83 miles 5;700 pounds; 59 miles; 5;400 pounds 44 miles 5;100 pounds; 5;070 pounds 29 miles 5;000 pounds; 4;910 pounds 22 miles 4;900 pounds; 4;770 pounds 16 miles 4;800 pounds. It appears that there is a serious error in the FMS software or somewhere in the flight planning system. Whether it did not take the landing gear down into account or whether taking the landing gear into account triggered another mathematical error; it provided gravely wrong data which could have proved fatal. Fortunately; I looked at the fuel numbers and they didn't look right to me so I did my own math. If we had waited for the low fuel warning; we would most probably not have had sufficient fuel to land at an airport. If you look at the numbers above; the FMS showed for example that we could fly 22 miles; gear down; with 10 lbs of fuel. Or we could land with more fuel than we had on board.

Google
 

Original NASA ASRS Text

Title: An EMB170 was maintenance ferried [with] the gear down with 13;300 pounds of fuel for the 375 mile flight; but inflight fuel burn was actually 7;000 pounds per hour so the flight landed at a divert airport well short of its destination with 4;800 pounds.

Narrative: We were ferrying the aircraft after landing gear malfunction on the previous leg. In preflight planning; I checked the Dispatch release. The operation modes for climb; cruise; and descent were 250/GRDN. At pushback; the FMS indicated that we had 13;300 LBS of fuel on board with a landing fuel of 7;800 LBS. About 20 minutes into flight; I checked our fuel burn rate. We were burning 7;000 LBS of fuel per hour. Meanwhile; the FMS was still showing that we would land with 7;800 LBS of fuel. We updated the fuel on board in the FMS [but there was] no change. Something didn't add up. I did the mental math; we were burning 7;000 LBS of fuel per hour; we had 8;500 LBS of fuel on board and we had more than an hour to go. I did the math again; how could this be? Not only would we not land with 7;800 lbs of fuel; at this rate; we could/would run out of fuel before reaching our destination. While the First Officer and I were working these numbers; Dispatch sent a message asking us how we were doing on fuel. I gave him the fuel update and asked him what the highest altitude was we could fly. He told us we could go to 20;000 FT but that the fuel numbers were for 12;000 FT. We climbed to 12;000 FT. The fuel numbers still did not look good to me. We started looking for the nearest suitable airport to land. I called Dispatch; we agreed on a diversion airport. When we were 16 miles from the airport; the FMS was showing fuel on board 4;770 LBS and the landing fuel 4;800 LBS; which is obviously not possible. Thankfully; we landed safely. Some example of the fuel numbers from FMS; fuel on board vs. miles to go vs. landing fuel: 13;000 LBS 375 miles 7;800 LBS; 6;850 LBS 99 miles 6;000 LBS; 6;400 LBS 83 miles 5;700 LBS; 59 miles; 5;400 LBS 44 miles 5;100 LBS; 5;070 LBS 29 miles 5;000 LBS; 4;910 LBS 22 miles 4;900 LBS; 4;770 LBS 16 miles 4;800 LBS. It appears that there is a serious error in the FMS software or somewhere in the flight planning system. Whether it did not take the landing gear down into account or whether taking the landing gear into account triggered another mathematical error; it provided gravely wrong data which could have proved fatal. Fortunately; I looked at the fuel numbers and they didn't look right to me so I did my own math. If we had waited for the low fuel warning; we would most probably not have had sufficient fuel to land at an airport. If you look at the numbers above; the FMS showed for example that we could fly 22 miles; gear down; with 10 lbs of fuel. Or we could land with more fuel than we had on board.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.