B737 captain reported the aircraft climbed 400 feet while on altitude hold while they were looking f...

Attributes
ACN	1399685
Time
Date	201610
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Route In Use	Vectors
Flight Plan	IFR
Aircraft 2
Make Model Name	Medium Large Transport
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	Altitude Hold/Capture
Person 1
Function	Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 99 Flight Crew Total 21000 Flight Crew Type 4000
Events
Anomaly	Aircraft Equipment Problem Less Severe Conflict Airborne Conflict Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance

Narrative:

While in cruise flight we were being given vectors at FL200; center called traffic at our 11 o'clock position at FL190. For about 90 seconds the first officer and I scanned the horizon for the [aircraft]. Then the altitude alert system chimed indicating we were (+400 feet) off altitude set in our cruise panel window. I gently lowered the nose so as not to cause the other aircraft TCAS to go off based on our descent rate. I then re-engaged the altitude hold.after speaking with our mechanics on the ground this is not the first or second time this aircraft has been written up for such an event. Maintenance always writes up our aircraft as unable to duplicate. It is my fault for not having monitored our altitude while looking for traffic.

Original NASA ASRS Text

Title: B737 Captain reported the aircraft climbed 400 feet while on altitude hold while they were looking for traffic; a repeat gripe for this aircraft.

Narrative: While in cruise flight we were being given vectors at FL200; Center called traffic at our 11 o'clock position at FL190. For about 90 seconds the First Officer and I scanned the horizon for the [aircraft]. Then the altitude alert system chimed indicating we were (+400 feet) off altitude set in our cruise panel window. I gently lowered the nose so as not to cause the other aircraft TCAS to go off based on our descent rate. I then re-engaged the altitude hold.After speaking with our mechanics on the ground this is not the first or second time this aircraft has been written up for such an event. Maintenance always writes up our aircraft as unable to duplicate. It is my fault for not having monitored our altitude while looking for traffic.

Data retrieved from NASA's ASRS site 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.

B737 captain reported the aircraft climbed 400 feet while on altitude hold while they were looking for traffic; a repeat gripe for this aircraft.