Overview
• Operator: United Airlines
• Aircraft Model: Boeing 737-9 MAX
• Registration: N37560
• Flight Number: UA-329
• Route: Denver (DEN) to Boston (BOS)
• Date: 30 September 2023
• Occupants: 167 passengers, 6 crew
• Casualties: None reported
• Nature of Incident: High-speed rejected takeoff due to right main gear fire and overheating
Accident Summary
A United Airlines Boeing 737-9 MAX, operating as flight UA-329 from Denver (DEN) to Boston (BOS), rejected takeoff at high speed on runway 16R due to abnormal acceleration and an observed fire on the right side of the aircraft.
Denver Tower immediately informed the crew that flames were visible from the right engine area, prompting the pilots to abort takeoff and stop the aircraft on the runway. The runway was closed, and emergency services were dispatched.
The FAA’s initial report incorrectly stated that the fire was under the right wing, while pictorial evidence and later NTSB findings confirmed extensive fire damage to the right main landing gear area.
A replacement Boeing 737-9 MAX (N37541) was dispatched, reaching Boston with a 5-hour delay.
On February 26, 2025, the NTSB released its final report, confirming that overheated brakes due to an extended taxi at higher power settings caused a wheel fire during takeoff, resulting in substantial damage to the aircraft.
Sequence of Events
Pre-incident Conditions:
• The aircraft had just returned from maintenance, prompting additional pre-flight checks by the crew.
• Due to high temperatures (31°C) and near-maximum takeoff weight (172,800 lbs), the aircraft exceeded the allowable takeoff weight for runway 16R.
• The original flight plan had been calculated using a 29°C temperature, but the rise to 31°C required a weight reduction.
• Dispatch initially removed 1,000 lbs of cargo but this was insufficient.
• A second revision removed 8 passengers to further lower weight.
• A third revision suggested burning off 1,000 lbs of excess fuel through an extended taxi rather than offloading more cargo or passengers.
Taxi & Brake Overheating:
• The aircraft performed a long, slow taxi at elevated power while applying additional braking to increase fuel burn.
• After reaching the runway, the aircraft held position for 10-15 minutes with parking brakes engaged to further burn fuel.
• The Boeing 737-9 MAX does not have a brake temperature monitoring system, unlike other Boeing models.
• The crew had no indication that the brakes had overheated before takeoff.
Takeoff Roll & Fire Incident:
• Upon initiating takeoff, the captain noticed abnormal acceleration and immediately rejected takeoff.
• Denver Tower observed smoke and flames from the right-hand side of the aircraft and notified the crew.
• The aircraft came to a stop on the runway, and emergency services were dispatched.
• The fire appeared to self-extinguish before emergency services arrived, though significant damage had already occurred.
Emergency Response & Passenger Handling:
• ARFF (Aircraft Rescue and Fire Fighting) sprayed both main gear assemblies with fire retardant.
• The captain made multiple PA announcements to keep passengers informed.
• After consulting with emergency responders, the crew decided against an immediate evacuation.
• Passengers deplaned via air stairs and were transported to the terminal by buses.
Investigation & Findings
Final NTSB Report (February 26, 2025):
The probable cause of the accident was:
• Overheated brakes due to an extended taxi at high power settings in an attempt to burn off fuel, resulting in a wheel fire during takeoff.
Contributing factors included:
1. Lack of a brake temperature monitoring system on the Boeing 737-9 MAX, which prevented the crew from detecting excessive brake temperatures.
2. Dispatch’s decision to reduce weight by fuel burn rather than removing more passengers or cargo, leading to prolonged braking.
3. Holding at the runway for 10-15 minutes with the parking brake engaged, exacerbating brake heat buildup.
4. The aircraft had just come out of maintenance, but no known brake issues were reported pre-flight.
Post-accident Damage Assessment:
• Right main landing gear wheel rims were ground down due to extreme heat.
• All four main tyres blew out, with two completely separating from the wheel assembly.
• Wheel fragments struck the fuselage, causing minor impact damage.
• Debris was found embedded in the inner face of the engine inlet acoustic panel.
• The No. 2 engine nacelle scraped the runway.
• Heat damage to multiple wing-to-body fairing panels.
• Right horizontal stabilizer sustained structural damage.
Analysis & Safety Recommendations
For Flight Operations & Airlines:
• Avoid prolonged taxiing at high power while braking to reduce weight—alternate weight reduction strategies should be prioritized.
• If fuel burn-off is necessary, aircraft should taxi without excessive braking or hold times.
• Boeing should consider implementing a brake temperature monitoring system on the 737 MAX fleet, similar to other Boeing aircraft.
• Flight crews should be trained to recognize risks associated with extended taxi and brake heating, especially in high-temperature conditions.
For United Airlines & Dispatch Procedures:
• Weight reduction strategies should favor offloading cargo or passengers over excessive fuel burn via taxiing.
• More proactive monitoring of weight changes due to environmental conditions (e.g., temperature fluctuations) should be incorporated into dispatch decision-making.
• Dispatchers should be trained to avoid suggesting extended taxi operations as a primary means of weight reduction.
For Airport & Ground Operations:
• Denver ATC handled the emergency well, but additional training could enhance real-time monitoring of taxi operations for signs of overheating.
• Runway hold times should be minimized in hot weather conditions to reduce brake heat accumulation before takeoff.
Weather Conditions at the Time (METAR Reports for KDEN)
• Wind: 190° at 8 knots
• Visibility: 10 miles
• Temperature: 31°C (88°F)
• Altimeter: 29.92 inHg
Conclusion
This incident highlights the dangers of excessive brake heating due to extended taxiing at high power settings, particularly in hot weather. The lack of a brake temperature monitoring system on the Boeing 737-9 MAX was a critical factor, as the crew had no way of knowing the extent of brake overheating before takeoff.
The NTSB findings emphasize the need for better weight management strategies and improvements in flight dispatch decision-making to prevent similar incidents in the future.
Disclaimer
“This report is based on available information as of 27 February 2025. While every effort has been made to ensure accuracy, the completeness of the details cannot be guaranteed. If you are the rightful owner of any referenced materials and wish them removed, please email takedown@cockpitking.com.”
